2D Rotate tool always resets rotation to zero.

[rgrams]

Operating system or device:
Win 7, 64 bit. Godot 2.0 RC1. But I'm assuming it works this way for all platforms and all versions.

Issue description
When rotating nodes in the 2D viewport, the rotation always snaps back to zero first, which makes it confusing to use and nearly impossible to make small, precise changes.

I expect it to add an offset to the current location based on your mouse position. (this is how the 3D rotate tool currently works.)

P.S. I like the new bug report template.