Be able to create prerelease packages and publish to custom feeds

New build parameters:
- nugetpublishurl: The feed to publish too. Uses default if not specified
- nugetprerelease: Prerelease prefix.

Example: If "-nugetprerelease=dev" is used as parameter when executing
build.cmd a prerelease package will be created with a version on the format:
version + "-dev" + DateTime.UtcNow (unfortunately seconds had to be left
   out as Fake only supports an int)
For example: 1.6.4-dev14081911