This commit simplifies Fragment by replacing the request/response channels
with a `sync.Mutex` to restrict access. This also simplifies the
`FragmentContainer` code which can largely be removed now since most of
the methods are simply wrappers around `GetFragment()` and `Fragment`.
This commit moves the index package to the root. Because pilosa
is an index at its core, it's redundant to have an index
subpackage. It also provides better naming such as `pilosa.Bitmap`
instead of `index.Bitmap`.