Maintenance release

• Maintenance release (no libsass upgrade)
• Add unit tests for included plugins
• Changed plugin locations and auto loading
• Implement support for HRX spec tests
• Fix issue with newer ExtUtils::CppGuess
• Update ppport.h and adjust makefile
• Update sassc plugin to latest version
• Update img-size plugin to latest version
• Add loading of alternate spec output file names
• Fix spec test by trimming trailing white-space
• Add appveyor for windows/strawberry perl testing