fixes and enhancements to psipshon install script
fixes and enhancements to psipshon install script

* don't clone repository if the directory already exists
* eliminate duplicated lines copying the server file
* don't compile ssh client if the file already exists
* fix syntax of line checking if a virtualenv is activated
* fix psiphon directory path
* remove x flag
* remove commented lines for running psiphon
* replace source command by dot
* add variables for psiphon repo url and ooni virtualenv path
* fix identation

0xPoly/ooni-probe,0xPoly/ooni-probe,0xPoly/ooni-probe,0xPoly/ooni-probe