Merge pull request #150 from fzerorubigd/develop

Detect blocked site by custom HTTP error code.
Decrease maximum idle server connections to 5.
Close idle upon too many open fd error.
Run the job when it's loaded.
Merge pull request #157 from luosheng/launchd

Ensure cow is loaded and run at startup in OS X Yosemite
Refactor proxy selection.
New load balance strategy: latency.

Select proxy with lowest connection latency.
Use Go 1.3 for Travis CI.