wellington formula

* brew compiles and installs a static libsass
* pkg-config for reading the correct path
* updated for 3.1 instructions
* update for libsass 3.1beta2
audit warnings
pipe input into wt
enable cxx11 in the stage and install def
specify g++ external linker for ML
print build commands, use ML symbol

* add libsass revision
enforce c++11 at cgo
wellington: add 0.5.0 bottle.