Nuget post build config fixes

- Use $(ConfigurationName) variable so solution will build for debug and
release
- Use nuget.exe that's bundled with the project
Add extensions for HEAD calls

basic support for HTTP HEAD method
Merge pull request #6 from OldTownIT/head-support

Add extensions for HEAD calls
Merge pull request #5 from OldTownIT/nuget-postbuild-fixes

Nuget post build config fixes
only package release builds
update Flurl.Http to 0.1.3