Added status messages for the Build command
Removed project references from the Sass library
Added a SassCompiler submodule to handle Sass compilation

- Defined a new command in the MailGen app to allow compiling Sass for emails

- Created the SassCompiler submodule. This sets the various options to pass to gulp-ruby-sass, including setting the load paths so that the standard Sass library can be @imported
Moved the Sass library into a "mailgen" subdirectory

- This allows @import rules like '@import "mailgen/screen"', reducing ambiguity and the risk collisions between project and library file names