Attempt to Fix all warnings from Sass.

Hi all, This is an attempt to fix different kinds of warnings form 

* Circular dependency loading
* Un- initialized variable.
* Use of File.exists? instead of File.exist?
* Parenthesis around ambiguous arguments, etc.

Not sure what the policy is about this. Just an attempt to clear out some warnings I see on Sinatra builds. (https://travis-ci.org/sinatra/sinatra/jobs/19503469)