nunit.framework.dll causes conflicts with unit tests

[seladavis]

If you use Unity's built-in NUnit to write your own tests, nunit.framework.dll in the following folder causes a conflict at compile-time:

https://github.com/kimsama/Unity-QuickSheet/tree/master/Assets/QuickSheet/GDataPlugin/Editor/Google

I do not believe this dll is necessary to ship anymore and can be removed. Related to #23.

[kimsama]

Thank you for the report. Fixed.