Schema versioning

[brettz9]

There are a bunch of minor fixes or refactoring, testing, and doc improvements in this PR, but the main change in this PR is versioning of schemas. It is backwards-compatible, with the exception that now it will delete unused indexes by default (which I think should be fairly safe and fits in with the behavior that we auto-delete unused stores) and it will also destroy and rebuild stores if they are respecified with different options (e.g., a different keyPath). The README ought to be helpful in visualizing the new schemas (and schemaBuilder) formats.

• Fix: Ensure return after rejection (in get and count);
• Fix: Better type-checking for bad schema objects;
• Fix: Polyfill newVersion for PhantomJS
• API change (breaking)/Feature: Add clearUnusedIndexes option (defaults to true);
• API change (breaking)/Feature: Destroy and rebuild stores or indexes if present with different options;
• API addition: Support a schemas object keyed to version with values
  as schema objects. (Utilizes own IdbImport class, relying on own fork of idb-schema at least for now.);
  idb-schema callbacks will be passed db.js Server as second argument
• API addition: Support a schemaBuilder callback which accepts an
  idb-schema object for incremental,
  versioned schema building and whose addCallback method will be
  passed an enhanced upgradeneeded event object with a Server object
  as a second argument for making db.js-style queries
  (e.g., to modify store content), albeit with certain limitations.
  Along with schemas, addresses issues Update/database migration #84/There is no way to test if the db.js is upgraded #109
• API addition: Support a clearUnusedStores property to conditionally avoid deleting old stores.
• API addition: Add db.del alias of db.delete
• API addition: Add del() alias on Server for parity with idb-batch (likely to use or adapt for multiple change transactions);
• Feature: Change schema (and allow for schemas) to differentiate between "mixed", "whole", "idb-schema", and "merge" types with schemaType option (defaulting to mixed for schema and whole for schemas, maintaining prior behavior in backward-compatible manner);
• Docs: Update version and schema to take schemaBuilder into account
  (and document schemaBuilder).
• Docs: Expand on key behavior examples;
• Docs: Indicate that key is optional too
• Docs: Add mention of missing limit on modify
• Refactoring: Use const where possible;
• Refactoring: Change variable placement including reordering static variables according to type (built-in alias, IDB, immutables, cache)
• Refactoring: Avoid try-catch for brevity if will throw in sync body of Promise (but denote which lines may throw)
• Refactoring: simplify transaction building
• Refactoring: Add lang attribute for Atom linter-jade package;
• Code comment: preventDefault not only for Firefox.
• Code comment: Clarify how old connections may be closed
• Testing: Schema building tests
• Testing: Add tests for caching
• Testing: Add test:local and phantom on package.json scripts
• Testing: Fix server-handler test (expect specific error)
• Testing: Fix test-worker to deal with multiple babel-polyfill instances
• Testing: Remove redundant bad-args test
• Testing: Unregister service worker

[brettz9]

Any plans to review? :)