Add .less code preprocessor option

Enable passing a `pre` option to preprocess source .less code before parsing. Its value is a function that receives the source code and the Connect request object as parameters, and must return preprocessed source code. This can be used for example to namespace css output depending on request parameters.
Change option to `preprocessor` and cleanup code
Add preprocessor option description and example to README
Merge branch 'master' of github.com:njoyard/less.js-middleware into issue49

Conflicts:
	readme.md