wellington formula

* brew compiles and installs a static libsass
* pkg-config for reading the correct path
* updated for 3.1 instructions
* update libsass for the 3.1beta2 release https://github.com/sass/libsass/issues/697
> It's far simpler to vendor in home brew, sub modules are just a pattern
> that does not work for this use case.
>
> If I upgrade Wellington then I'll upgrade the libsass revision too if
> that's even necessary. This is a good compromise until libsass is stable
> enough to depend on the libsass formula.

If the process with upgrading the formula relies on you doing it then this should be in a tap instead. Otherwise we need to work out an upstream solution that doesn't rely on you knowing the revision and being the one to upgrade it as we don't have external maintainers for packages in Homebrew core.

> For the test what is wrong with piping? The tmp directory is sent to stdout
> so the output of my test will be non deterministic and difficult to asset
> equity. Stdin is more efficient (no generated files) and easier to test
> (deterministic output from stdout of Wellington). If this is still a
> problem, I'll have to write string replace code to strip the randomly
> generated temp directory from the output.

The