added support for proxying URLs

thanks to
https://github.com/kevindente/testem/commit/8dfaa4319c6f15733dd898666a016f3b2d96d84a
increase proxy priority and add documention
Merge branch 'master' of github.com:ghedamat/testem into ghedamat-master
Merge branch 'ghedamat-master'
0.6.18