Update nuget spec with new version and release info.
Update nuget spec with new version and release info.

Bump to 2.0 alpha.

neutmute/UnitsNet,anjdreas/UnitsNet,BoGrevyDynatest/UnitsNet,maherkassim/UnitsNet,maherkassim/UnitsNet,BrandonLWhite/UnitsNet,anjdreas/UnitsNet,Tirael/UnitsNet,BrandonLWhite/UnitsNet