job: save the real exception when unpickling fails

We raise our own exception which hides the real error (often an ImportError),
making it difficult to see what happend. Instead, save the original exception
too.
Make inner_exception optional, and use new except syntax.
Patch the connection instances.

This patches the connection object (which is either a StrictRedis
instance or a Redis instance), to have alternative class methods that
behave exactly like their StrictRedis counterparts, no matter whether
which type the object is.  Only the ambiguous methods are patched.  The
exhaustive list:

- _zadd          (fixes argument order)
- _lrem          (fixes argument order)
- _setex         (fixes argument order)
- _pipeline      (always returns a StrictPipeline)
- _ttl           (fixes return value)
- _pttl          (fixes return value)

This makes it possible to call the methods reliably without polluting
the RQ code any further.
Bump version to 0.3.5.
Add release date.
Prepare for 0.3.6.