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
I mean relying on our formula to define what revision is used. If a new version of wellington is released how will we know what `sass` revision is required? That's why I'd like it if this can be done as a submodule or vendored so that it's defined by the wellington buildsystem rather than ours. Otherwise it might be better to host this formula in a tap yourself.