Add allow_net_connect! connection method

Also adds disable_net_connect! to go back to mocked connection.
Add existing mocks to error messages

Helps debugging updating mocks with new data.
Merge pull request #1 from dplummer/more-error-message

Add existing mocks to error messages
Merge pull request #2 from dplummer/allow-net-connect

Add allow_net_connect! connection method
Move class saving to disable

This means if you override the Connection after the gem is loaded, then
call disable_net_connect!, your override is saved.
Merge pull request #3 from dplummer/allow-net-connect

Move class saving to disable
Change the connection object too
Merge pull request #4 from dplummer/allow-net-connect

Change the connection object too
version bump v0.2.2
fix json_api_client version dependency in gemspec