Possibility to define a prefix

Added possibility to define a prefix for the cached files.
Fix typo
Update README.md
Update .travis.yml to send coverage to scrutinizer
Update README.md
Merge pull request #33 from marclaporte/patch-1

Fix typo
Merge pull request #31 from Ninos/prefix

Possibility to define a prefix
Enable overriding LESS variables set in files

This patch switches the order of parsing files from

 apply variables - parse - get css
to
 parse - apply variables - get css

in the lessc class.
Merge pull request #36 from s7eph4n/master

Enable overriding variables set in LESS files when using lessc adapter class
Update composer.json

Prepare for packagist.
Update composer.json
Update README.md
Merge remote-tracking branch 'Less-PHP/master'
Added unit test for :extend(input[type=text]).