add file override support to nuget params
fix some small typos
cleanup and code review. Target and Exclude are optional according to the spec, so make it so.
Merge pull request #390 from baronfel/nugetFileParams

Allow explicit file specification on the NuGetParams Type.
Merge branch 'develop' of https://github.com/fsharp/FAKE
Added release notes - references #390
Bump version to 2.13.3