Refactored compiler submodules

- To improve code organisation, decided to move both of the compiler submodules (SassCompiler and Build) to a Compilers subdirectory

- Renamed the Build submodule to HtmlCompiler to better reflect its purpose, and refactored methods and calling code accordingly
Renamed scaffold.js to Scaffold.js for consistency
Moved HtmlCompiler gulp options into properties
Moved the Scaffold submodule into a Commands subdirectory
Consolidated the compilation commands

- HTML and Sass compilation are not likely to be required in isolation - instead, a single command should run all tasks required to build an email from source

- Removed the "sass" command from the app, consolidating it into a single "build" command which now performs both HTML and Sass compilation in a single step (delegating the work to the relevant compiler objects)
Added a Watch submodule to handle the watch command

- The Watch submodule uses gulp's watch command to watch an email's source directory, and updates the user's browser with livereload when changes are detected

- Added a getEmailFromName method to the EmailHelper constructor to allow retrieval of a single Email object by name