Download page updates.

- updated latest stable version to 1.2.1
- updated links to point to ares
updated conf.py to reflect latest stable version
Updated quickstart to reflect latest versions
Add build instructions to README
README updates
Renaming README.txt to README.md
Updating versions.

pom version to 1.4-SNAPSHOT and geotools dependency to 11-SNAPSHOT.
Updated repository settings.

- Replacing opengeo repo with boundless
- Added osgeo repository