Lodash 4 compatibility (#585)

Resolves #561. Summary of changes:

 - It is now possible to run the entire test suite with Lodash instead of Underscore by passing the `--lodash` option to `karma start`. Still runs with Underscore by default. (Tip for Yarn users: you can just do `yarn test --lodash`).
 - Feature detection of `_.any` is used to determine whether the `_` factory argument is Underscore or Lodash. In the latter case, some aliases are created on `_` in order to keep the code changes to a minimum.
 - Everywhere a third `context` argument was passed to functions like `_.each` and `_.map`, `_.bind` is now used instead.
 - Travis build runs both the Underscore variant and the Lodash variant of the test suite (and both pass).
 - Travis config updated to use Node version 8 instead of 4, in order to resolve compatibility issues with the force-updated `npm` command line program.
 - `index.html` updated to reflect that Lodash works, too.

Note on the `CONTRIBUTING.md` and the `.editorconfig`: these files tell me that JavaScript files should be two-space indented, but the JavaScript files I edited were already tab-indented. I decided to go with the existing practice.

.travis.yml

@@ -1,11 +1,12 @@
 language: node_js
 node_js:
-  - "4"
+  - "8"
 before_install:
   - npm install -g npm
-  - npm install -g karma-cli
+  - npm install -g karma-cli@1
 script:
   - npm test
+  - karma start --single-run --browsers PhantomJS --lodash
 sudo: false
 cache:
   directories:

CONTRIBUTING.md

@@ -20,7 +20,7 @@ Before you submit your Pull Request ensure the following things are true for you
     - How to setup [ESLint](http://eslint.org/docs/user-guide/integrations)
 2. Your changes are branched off of `master`
 3. You have added a test case for your changes or updated an existing test case
-4. All test cases are passing
+4. All test cases are passing, both with Underscore and Lodash
 
 # Running Unit Tests
 You can run tests using PhantomJS (headless) or a web browser (Chrome)
@@ -29,3 +29,5 @@ You can run tests using PhantomJS (headless) or a web browser (Chrome)
 - To run tests in a browser:
     1. Run `karma start --single-run`
     2. Open Chrome and connect to the url it outputs
+
+You can test with Lodash instead of Underscore by passing the `--lodash` option to `karma start` or `yarn test`.