Add create_index / drop 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

* misc.
  - [ ] convert PyTables `AssertionError` to `ValueError` or `TypeError`
  - [ ] find a less verbose example for the `create_index` and `drop` docstrings
the sql backend is much less verbose