File GET API, convenience methods and bug fixes

These changes were created in order to import items from Omeka into FromThePage.

Features:
f2e23504053234ff4f8df09d9a4fbf8576970085 New File model
678fe8beeb734db4e3fd7ba84ae5cd51a9a813e7 ActiveRecord-style relationship methods
6979b70e3ea472d4327d5cbde60316aed0459228 ditto
3240df8fb4b8c17f709924072feb4a1df7337b06 Don't throw method_missing for attributes with no values.


Bug fixes:
5b205e5d301f424a216a64ebf29831559701880a Fixed a bug in which query parameters to GET calls were getting lost.
f7ef640875b9a56ff87b7bb1430e0c736ca89f50 Removed redundant code within method calls.

Refactoring:
898ba32eaf37f2a2846d66f1fe1b1c28cb7c9c02 Externalized the Omeka credentials used by the test code