Improved install.sh

1. Send output of sourcing .rc files to /dev/null.
2. Fixed postgres version checking as some installations return 2 liners
3. Better handling of virtualenv. Handles if it's installed but not enabled.
4. Fixed checking for npm binary
5. Fixed PG_HOST=localhost potential bug on some postgres installations
6. Checks for a postgres user before attempting to create one to reduce error messages.