Bug fixes in the subscribing mode.

Thanks doujiang for the patch in #38.

* bugfix: the unsubscribe/subscribe commands could not be called after read_reply() returned "timeout".
* bugfix: we incorrectly allowed reusing redis connections in the subscribed state.
fixed a test case that could fail due to machine differences.
bumped version to 0.20.
removed the years from the in-source copyright.
suppressed a false positive in libdl.
fixed a test case for redis 2.8.x.
tested the select() command also.
doc: added a new section "Check List for Issues".
doc: mentioned the maxclients redis parameter too.
doc: mentioned the "bad request" error in the "Limitations" section.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	lib/resty/redis.lua
	t/pubsub.t