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
Add a comment here pointing to https://github.com/wellington/wellington/blob/master/.libsass_version and explaining that they should match, thanks.