several changes to the installation section:
several changes to the installation section:

- removed old installation text
- added section on marvin SDSS dependencies and SAS_BASE_DIR
- added section for FAQ about installation

sdss/marvin,albireox/marvin,bretthandrews/marvin,sdss/marvin,albireox/marvin,bretthandrews/marvin,albireox/marvin,sdss/marvin,sdss/marvin,albireox/marvin,bretthandrews/marvin,bretthandrews/marvin