use virtualenv

Instead of using sudo to install everything system wide, I prefer to use python's virtualenv.

It's also better to set CFLAGS/LDFLAGS instead of using brew link --force for keg-only formulas (openssl here) since linking keg-only formula can cause problems.

I also dropped the linking of sqlite3 since that has similar issues as linking openssl.  If we really want the brew version of sqlite3 on the path, maybe it would be best to link it into ./env/bin/.  What was the motivation for that link?