use travis-ci to create a nightly build

[zhengwy888]

not sure if this could be useful. travis ci can upload compiled zip so people can skip the compilation themselves. I could try to make a working .travis.yml

[Sunappnio]

The author already uses travis-ci in his project shadowsocks-nacl. Personally I think this app is mainly for chromebook users, who are supposed to be skilled at compilation.

[nekolab]

Thanks for your advice~ Actually it's already in my plan, but I cannot provide the exact time due to my personal reason As always, pull request is welcome

PS: I saw some tutorial about how to compile and install this project on chromebook, requires enable develop mode to install nodejs and coffeescript, it's a bit complicated. If you have a PC/MAC, you can compile this project on it, install nodejs on PC/MAC will be easier than on a chromebook, then copy the compiled files to chromebook and load it in chrome://extensions page.