feat(InboxAPI): support setting HTTP headers for all requests, and withCredentials flag

These can be specified at runtime, or during initialization of InboxAPI. Ideally, one should use
the exposed API for doing this.

Closes #25
Closes #24
Closes #7
chore(.jshintrc): make jshint happy
feat(angular-inbox): support configuring HTTP headers and withCredentials

New methods are exposed similar to InboxAPI's runtime methods for specifying request headers and
the withCredentials flag, in order to support authentication.