PeanutButter is updated to NUnit 3

NUnit (https://github.com/nunit/nunit) has been updated to version 3 for a little while now, so I thought it best to move PeanutButter forward as the test utilities which use NUnit for assertions and such have required the user to explicitly install NUnit 2.6.4.

In particular, this will affect any new installations or updates to projects using the following:

•  PeanutButter.TestUtils.Generic 
•    Which includes PropertyAssert
• PeanutButter.TestUtils.Entity
• Which includes the helpers around testing database persistence of your entity models

What this means is:

• If you’re not depending on any of these PeanutButter modules, of course nothing changes for you. Though I’d still recommend moving forward to NUnit 3 at some point when you have some time. There are some changes and some things to deal with, so don't rush it.
•  If you have a project which is currently stable and not in active development, don’t bother updating anything unless you really want to stay up-to-date.
•  If you’re going to install-package PeanutButter.<something>, you may find that you’re automatically updated to NUnit 3. Mostly, it’s not a problem and the differences are easy to work around. If you get a PeanutButter package with version 1.1.x instead of 1.0.x, you’re going with NUnit 3. If you don’t want to go this route, install the last 1.0.x version: 1.0.155. You may have to specify an NUnit version to install, ie:
  install-package NUnit –project <your project> -version 2.6.4.
  I used to make my packages not depend on specific versions of anything, but I’ve recently started adding minimum versioning to packages to try to alleviate some of this headache.