Add support for ghq

`ghq` is a tool which is developed by @motemen and allows us to manage cloned repos easily.

http://motemen.hatenablog.com/entry/2014/06/01/introducing-ghq

I'm convinced that it must be useful gem-src uses ghq instead of its original cloning strategy.

output:

```
$ gem i glint
Fetching: glint-0.0.2.gem (100%)
/Users/usr0600239/bin/ghq
     clone http://github.com/kentaro/glint -> /Users/usr0600239/src/github.com/kentaro/glint
       git clone http://github.com/kentaro/glint /Users/usr0600239/src/github.com/kentaro/glint
Cloning into '/Users/usr0600239/src/github.com/kentaro/glint'...
remote: Reusing existing pack: 185, done.
remote: Total 185 (delta 0), reused 0 (delta 0)
Receiving objects: 100% (185/185), 20.76 KiB, done.
Resolving deltas: 100% (61/61), done.
Successfully installed glint-0.0.2
1 gem installed
```