format_import_directive method

Usage:
  Asphalt::Utils.format_import_directive("partials", "foo")

  #=> "@import 'partials/foo';"
output import statement as sass or scss
Less robotic comments for template files
last_import_statement method

Retrieves the last import statement in a sass file
Refactor specs

* Break out specs to its own file
* Add Rspec rake task
* Move temp directory methods to spec helper
* Do not create a temporary directory if it exists
* Clear temp directory instance variable after deleting directory
Add `.create_sass_file` method
Add `.add_import_statement` method to Asphalt::Utils
Don't output temp directory filepath
Add `update_file!` method
Remove whitespace from strings in specs

The whitespace at the beginning of the line is significant.
Who would've thunkit?

https://stackoverflow.com/questions/3772864/how-do-i-remove-leading-whitespace-chars-from-ruby-heredoc
Add `generate` command to CLI
Fix import statement formatting with directories

* Prevent leading slashes in import statements if the directory array is
  empty