Add Awk Support

I know that some efforts have been made to add [AWK](http://en.wikipedia.org/wiki/AWK) support ([#164](https://github.com/github/linguist/pull/164) and [#275](https://github.com/github/linguist/pull/275)), but here comes another try.

AWK is a handy text processing language which [a lot of people still work with](https://github.com/search?q=extension%3Aawk). Since pygments has a lexer for it, it seems quite reasonable to have it supported by GitHub.

have chosen [#6666cc](http://www.colorhexa.com/6666cc) as its color. and followed pygments' naming convention `Awk` (instead of all-capitial `AWK`)