Update Scripts/BumpVersion.ps1

Support bumping major, minor, patch or setting new version explicitly

Parameter sets:
-m|bumpMajor
-i|bumpMinor
-p|bumpPatch
-v|setVersion {string}
Remove alpha from nuget spec.

Alpha/beta labels will be appended by continuous integration (TeamCity) instead.

This simplifies a few pain points:
* Starting release branch, must remember to update nuspec from alpha to beta
* BEFORE finishing release branch, must remember to remove beta from nuspec
* AFTER finishing release branch, must add back alpha label to new nuspec back-merged from stable

Without alpha/beta semver labels checked in to code, we only have to remember bumping the version when starting a release/hotfix branch, which is standard procedure.
Revert nuget spec to last release.
Remove duplicate line.