Add global_id_prefix

Allows for removal of `test_` from the begining of generated ids.  This
makes StripeMock act more like Stripe if need be.
Add failing tests for global_id_prefix [#73]
Improve global id prefix specs [#73]
passing specs for local, now onto StripeMock::Server...
global_id_prefix working for StripeMock::Server
Refactor & cleanup
Document new global id prefix feature
Version bump to 1.10.1.3