WIP: 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
These PyTables examples seem fairly verbose.  Most of the work is in setting up the example.  It's not immediately clear how they work.  I'm not sure exactly how to clean this up - perhaps make it more explicit what is setup and what is showing how `drop` works?  Perhaps there is a backend with less ceremony.