Refactor compile logic into a function

The idea here is to wrap .css compilation into a single function call
for ease of reading and maintenance.
Compile .less files on startup

From issue #7:

> Since lessc is not that comfortable to use (it doesn't take a
> directory as argument), it is nice if watch-less can compile .less
> files first, then compile them every time they are updated."
Merge pull request #9 from ChrisCummins/master

Compile .less file on startup