Add virenv support

Usage:
    1. add "let g:jedi#virtual_env_path='/path/to/go'" in vimrc
    2. enjoy

TBD:
    1. if not configured, an error will be reported
    2. space in path?
    3. need well tested

Note:
    1. must specified before /site-packages
    2. will import all *.egg in site-packages