Incremental Release

NEW:

• Allow multiple keys in EXISTS command (see #156 and #157);

FIX:

• Close RedisPool when connection to Redis failed (see #136);
• Add simple INFO command argument validation (see #140);
• Remove invalid uses of next()

MISC:

• Update devel.rst docs; update Pub/Sub Channel docs (cross-refs);
• Update MANIFEST.in to include docs, examples and tests in source bundle;