Remove coveralls badge from README.

It's not accurate, and it doesn't seem straightforward to fix.
Add readthedocs badge.
Add pypi badge.
Merge pull request #577 from enthought/feature/fiddle-with-badges

Fiddle with badges
Fix a typo in our mailing list address.
Merge pull request #578 from enthought/bugfix/fix-docs-mailing-list-typo

Fix a typo in our mailing list address.
Adds `{get,set}_base_comm()` functions for engines.
Cleanup and remove unused functions.

Also removes `get_nengines()`.
Merge pull request #579 from enthought/feature/get-base-comm-engines

Adds `get_base_comm()`
Split a `nonce` function out of `uid`.
Only define DISTARRAY_BASE_NAME one place.
Add more example links in docs.
Rerun features notebook.

There were errors in the stored output before.
Merge pull request #581 from enthought/feature/add-nbviewer-links-in-docs

Add nbviewer links in docs
Add some docstrings to proxyize.
Replace some hardcoded values.
Merge pull request #580 from enthought/refactor/nonce-and-basename

Refactoring: nonce() and DISTARRAY_BASE_NAME
Merge branch 'master' into feature/block-redistribution

Conflicts:
	distarray/globalapi/context.py