Improved conditional directives

* Removed `== true` to make some directives shorter.
* Turned `$condition != true` into `not $condition` to make some directives cleaner.
Used a ternary operator in place of a directive
Removed a useless pair of braces
Added a space before !default flags

As recommanded here: http://sass-lang.com/documentation/file.SASS_REFERENCE.html#variable_defaults_.
Added a missing semi-colon messing with syntax highlighter
Made some string concatenation a bit more elegant and readable

Not only does this change make it quite easy to change "pull--" in "pull-" or whatever, but it also makes everything slightly easier to read.
Removed useless pairs of braces when including parameter-less mixins
Normalized the use of space before opening curly braces
Fixed some incorrect spaces, linebreaks and indents
Merge pull request #76 from HugoGiraudel/master

Slightly improving overall code quality