Merge pull request #104 from rikoe/UpdatePackageRestore

Migrate to nuget automated package restore
Attempt to add TeamCity override of NUnit task
Use TeamCity version of NUnit task when available

The command-line params for the two tasks are different, have to specify
32-bit in different ways, so use a condition to decide which task to
use.
NUnit version needs to be included in TeamCity task

Otherwise it defaults to really old version and doesn't run all tests.
Specified version incorrectly