bigBed parser

[danvk]

There are a few things happening here:

• I've switched to using jBinary for parsing data structures. This eliminates the need for ReadableView and allows formats to be specified more declaratively.
• I require an HTTP server for running tests. This facilitates using test data files and eliminates the need to mock XHRs (so the FakeXHR class can be deleted).
• I've written a spec and parser for bigBed format with tests.

Fixes #27

[coveralls]

Coverage increased (+4.42%) to 69.23% when pulling a2e43f3 on bigbed into a2f4298 on master.

[ihodes]

LGTM!