NUNIT 3 support

[gpolunin]

Hello,

I have a situation when I need to support big amount of NBehave tests. They are blocking me from updating NUnit for the project.

Please review update of NBehave for NUnit3 support:
#17

Thanks.

[MorganPersson]

I will look at it.

[MorganPersson]

NUnit3 is supported in 0.7. There are pre-release nuget packages of 0.7 available.