Make gevent optional again

Currently when gevent is installed, solar will use it automaticaly, we need to make it optional even if it's installed.

CLI and solar-worker should turn on gevent support, while separate scripts shouldn't by default. To turn gevent on you should need to do one of following:
1) Import gevent before importing solar
2) Place something like `from solar import enable_gevent; enable_gevent()` in any of files