Added invoiced_at GET parameters, PUT action to mark entry as invoiced

Now users will be able to search for entries that have been invoiced within a specified date range, and can mark entries as invoiced outside of Freckle.
Use the proper URLs for Expenses
fixed indentation
added the 400 Bad Request status code
Wrote the first draft of the Import API specification.

The biggest issue I see here is how we handle uploading a file. Right now I have it specified as a standard HTTP POST with content-type: `application/x-ww-form-urlencoded` for simplicity's sake, but we should discuss it in more detail.
Wrote the first draft of the Invoice API Specification.
Added Description as a query parameter in the list action of Entry API
First drafts for Locales/Currencies, Projects and Grous, Tags, Users
Wrote first draft of project specs.
Wrote first draft of user Specification.
Wrote first draft of Current User Specification.
Cleanup.
Wrote helper method to return a simple JSON array for any Resource that we want.