Add create_index / drop_index functionality

closes #488

* `create_index`
  - [x] `sqlite`
  - [x] `pytables`: this is slightly different in that you generally don't name your index with this engine
  - [x] `pymongo.Collection.create_index()`
  - [x] need to investigate other backends that might support indexes
  - [x] test `unique` parameters where possible (sql and mongodb so far)

* `drop`
  - [x] `tb.Table`
  - [x] `pymongo.Collection`
  - [x] SQL
  - [x] HDF5
  - [x] CSV
Okay that makes sense. I couldn't decide whether I wanted to make everything conform to a SQL like interface (where indices *must* be named) vs other backends that don't force names on indexes.