Adds `targets` attribute to client Distribution object.
Adds `is_compatible()` to Distribution and maps, client-side.
Move uid and DISTARRAY_BASE_NAME to utils.
Add `apply` function which is analogous to IPython.parallel's apply.
Tests for new apply function.
Add return value flag.
Return function result by default.
Refactor argument handling.
Merge branch 'master' into cowlicks-da-apply
Have the toplevel Makefile clean the docs dir.
Refactor _make_intracomm() stuff.
Merge branch 'master' into feature/subset-ranks

Conflicts:
	distarray/dist/context.py
	distarray/dist/tests/test_maps.py
Fix import bug with renamings.
Remove obsoleted method.
Adds failing test for context.apply()
Fix apply and _generate_key.

Removes the 'key_' prefix from keys made with _generate_key

Removes the ability for users to create their own names for variables on
the engines. Instead a uid with the '__distarray__' prefix is generated.

Otherwise, if users can create arbitrailily named variables, we won't
know what strings represent proxy objects on the engines. Since user
named variables won't have the '__distarray__' prefix.
Fix tests to reflect changes to context.apply API.
Modify @kwsmith's regression test to reflect new API
return_name -> return_proxy
Add test for positivify.