Implement write support (create/update/delete)

[PandaWhisperer]

Currently, the gem is read-only. Creating, updating and deleting of entities is not supported. This is because the sample service we are using for all the test cases does not support any data modification.

This shouldn't be very difficult to implement, as it mostly follows basic REST semantics. It mainly needs test cases written, and it will require either a different service (for instance, the TripPinService from the OData Tutorial), or manually hacking together some VCR cassettes.

Documentation is available here:

• Create
• Update
• Delete

[natemacinnes]

I will be needing this for a project I am working on.
Happy to contribute :)