added database guide
added database guide link to TOC of guides
added link to GSOC 2013 in database guide
Merge branch 'master' of https://github.com/sunpy/sunpy into database-guide
[db guide] clarified usage of database (both local and remote is supported)
[db guide] clarified format of the URL parameter
Merge branch 'master' of https://github.com/sunpy/sunpy into database-guide
Merge branch 'master' of https://github.com/sunpy/sunpy into database-guide
added table of DatabaseEntry attributes with short description and how they are set
Merge branch 'master' of https://github.com/sunpy/sunpy into database-guide
Merge pull request #580 from derdon/database-guide

added database guide