Reset the collections hash on #reset.

Fixes #2234.
Fetch collection names agnostically regarding the data structure of config['collections']
Nil-out `@collections`  so `#collections` will re-compile
Don't pretend there is a collections setter
Don't read the collection if it's the data collection
Add a script/test file for :heart: and :money:
Always render collections, just don't always write them
Extract Collections metadata from site config
script/test should probably use bundler
NO MORE DATA COLLECTION I CAN'T HANDLE IT
Use Hash.new instead of hash literal
TEST THE COLLECTIONS
Add test for collections' new #to_liquid sturff
Run jekyll build in features (must give subcommand)
Use the 'output' key instead of 'write' for writing out collections' document files.
Merge pull request #2238 from jekyll/fix-the-collections