[Managed SDK] Fix Issue #229 (Empty IDs on Insert)

Comprehensive refactor of the ID handling process for Inserts and
Deletes. Manages the payload manually instead of depending on code you
don't control (JSON.NET) to do the work.
[Managed SDK] Update unit tests for Issue 229 fix

- Comprehensive refactor of the MobileServiceTable.* tests to handle
cases where null and string.Empty are valid string inputs.
- In many cases, the mock request and response JSON strings have been
replaced with manipulating a JObject. This is to guard against changes
to the JSON.NET serializer. However it has not been implemented
everywhere.
- Added new tags to better segment the unit tests. new tags are: insert,
update, delete, lookup