Add additional OS platforms to PlatformAttribute
Add 32 and 64 bit tests
Merge pull request #115 from nunit/platform-attribute

Merging it as is then. The support I added for 64-Bit-OS is .NET 4.0 and 4.5 only because it was trivial to implement. Before messing around with pInvoke on the other platforms, I'd like to see a solid need for it.