detailed instructions on quick installation with pip/virtualenv

Instructions for a complete installation with pip/virtualenv, including YUI.

Future improvements:
- [ ] bin/das_db_import to take db host/port from das config file (I'll do)
  * regarding the default MongoDB port, shall we have it in default config (etc/das.cfg) same as in default installation 27017?
- [ ] the older installation instructions below are slightly old/deprecated/messy, shall we remove/change them? (can wait)

P.S. there is 100 ways for virtualenv installation, I'm listing the simplest one, and if it doesn't work users may refer to their manual or see what's available in their distro.