Separates variables to variables.less file to allow overwriting

[filecage]

I stumbled upon this quite often now, been using skeleton-less in many projects. It's the same issue every time: I am unable to modify Skeleton's variables because they are packed in the skeleton.less.

My previous approach was to add the skeleton.less to my application repository rather than requiring it as dependency and modifying the file. For my latest project, I am using webpack with react and with a different approach of source file distribution, it is necessary to define skeleton as a remote dependency. I have created this PR to allow modifying the variables anyway.

Backwards compatibility is kept, importing skeleton.less will result in a complete skeleton-package, including variables and main rules.

This PR refers to issue #13, but does not resolve it.

We might discuss the filenames - while default-variables.less makes it pretty clear, it looks ugly to me. variables.less could be a bit... eye-friendlier. :)