Drop create index uri

A few changes around `resource` and `drop` and `create_index`

Move resource out of API to top level

    blaze/api/resource.py -> blaze/resource.py

Move `drop` and `create_index` from `compute/core.py` into `resource.py`

Unify API around `drop` and `create_index`

Add URI dispatched versions of `drop` and `create_index', e.g.

    drop('my_file.csv')