Updated linux installation instructions.

- Added instructions for creating MySQL database.
- Un-hard-coded virtualenv location.  (Debian puts it in /usr/bin.)
- Moved /usr/local/bin/ path check up to support `which virtualenv`.
- Added numpy installation.
- Correction: STATIC_ROOT_DIR -> STATIC_ROOT
Merge pull request #1756 from sethoscope/758ab16c0253a65121ff23386356ae28e101cf1c

Updated linux installation instructions.