Add an undo stack to SgConnection

[ndunsworth]

SgConnection needs to have an undo stack.

The stack should be push and pop-able.

The use case for this is a publishing process that interacts with shotgun through multiple phases. One process might not know what the another has done so other than raising an error there isnt much that a developer can do about reverting the additions/changes which did make it to Shotgun.

With an undo stack the publish process would be wrapped in a try statement, push the stack, and if an exception is raised call undo which would revert any modifications.