Update to Architecture intro file:
* Updated description of druid architecture components
* Added links from descriptions to actual component pages
* Removed Data-Flow page, which was a (mostly) redundant subset of other pages under Architecture)
* Moved non-redundant info from Data-Flow to other Architecture pages
* Updated info on how data actually flows in Druid 0.6
Merge pull request #287 from metamx/igalDruid

Update to Architecture intro file: