attempt to provide only healthy connections from the pool

Adds redis.selector, a module that provides the best selector strategy
available on the current platform. A redis.selector polls a socket to
provide two pieces of functionality:

1. Check whether data can be read from the socket. Prior versions of redis-py
provided this behavior with just select.select(). select() has lots of
limitations, most notably a limit of ~1024 file descriptors. Now that
better selectors are available, this should make can_read() faster and
able to accomodate more clients. See #1115 and #486

2. Check whether a socket is ready for a command to be sent. This doubles
as a health check. It ensures that the socket is available for writing,
has no data to read and has no known errors. Anytime a socket is
disconnected or hung up, data is available to be read, typically zero bytes.

ConnectionPool.get_connection has been modified to ensure that connections
it returns are connected and are ready for a command to be sent. If
get_connection encounters a case where a socket isn't ready for a command
the connection is reconnected and checked again.

TODO: more tests for this stuff. implement EPoll and KQueue selectors.

Fixes #1115
Fixes #486

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