Issue 181

This PR revises the msbuild script to handle engine and console as well as framework. The script is used by the Travice CI build and may also be used at the command line by developers.

Summary of key changes so far:

* Move scripts to the root
* Rename NUnitFramework.msbuild => NUnit.proj, runbuild => build
* Use Build rather than Rebuild target when building projects and rely on separate Clean to do a clean build.
* Stop using OutputPath: to direct the build to the build directory. Build is now done in the bin directory, just as when building with Visual Studio.
* Use net-x.x targets under Mono as well as MS windows to stay consistent with the solution.
* Script is working under Travis CI

The branch can be merged at this point but not deleted yet. The following still needs to be done:

* CI test does not include engine or console yet
* Silverlight test doesn't work
* Netcf is commented out - we will most likely remove
* Packaging is not yet tested
* Remove the old Nant scripts (UPDATE: Now done)
* Change to use of nunit-console and remove test-harness
* Script is not working under TeamCity - pending a decision about the use of TeamCity.
 
I'll do a further PR on this branch when the above is complete.