Adding some information on using pip and virtualenv
Update installation.rst

Added some detail on how to install in a virtualenv
Update installation.rst
Conflicts:

	docs/getting_started/installation.rst
Update installation.rst

Removed references to PIL and added instructions for installing Pillow globally using pip (because there is no .deb package for Pillow)
Update installation.rst

Oops
Update installation.rst

I assume this is what you meant by fixing the header markup? Thanks for reviewing this.
fixed too short title overline
fixed "overline too short" error
fixed build warning about duplicate virtualenv reference
fixed another build issue
Merge pull request #1908 from dgrant/installation-guide-virtualenv

Installation guide virtualenv