Add tests for empty-string prefix
Remove unused option in Store plugin
Alphabetise options
Provide emulation of PUT/DELETE in Store plugin

As requested in #180 (see also #172), this adds an `emulateHTTP` option to the
Store plugin, which allows Annotator to emulate PUT/DELETE for old webservers
that don't understand these methods. If `emulateHTTP` is true, then Annotator
will sent PUT and DELETE requests as POSTs, setting a request header to denote
the desired method:

    X-HTTP-Method-Override: DELETE

See [Backbone's `emulateHTTP`][1] for reference:

[1]: http://backbonejs.org/#Sync-emulateHTTP
Provide emulation of JSON post in Store plugin

As requested in #180 (see also #172), this adds an `emulateJSON` option to the
Store plugin, which allows Annotator to communicate with legacy webservers
that don't much like POSTS with `application/json` type payloads. Instead, the
data to be sent is JSON encoded client-side and is sent as a standard form
request (with content-type `application/x-www-form-urlencoded`) with the JSON
stored in the `json` key.

In addition, if `emulateHTTP` is also set, the Store will add a `_method` key
to the form, containing the same value as the `X-HTTP-Method-Override` header.