Add test case for server redirecting twice.
Fix pool manager urlopen() redirect at most once issue.
2012 -> 2013
CHANGES version bump.
CHANGES.
Add RequestError subclass to PoolError.

The former PoolError `__reduce__` method was broken as it assumed the `url`
attribute was set, when in fact it was not.

As per issue #174, this commit creates a `RequestError` subclass of `PoolError`
and makes `MaxRetryError` and TimeoutError` a subclass of `RequestError`.
Make HostChangedError a subclass of RequestError.
Make RequestException unpickleable and provide complete cycle (pickle/unpickle) tests.
At least, HTTP(S) proxy support works now...
Fixed proxy test
Inplemented CB's suggestions regarding code clarity. Fixed proxied Host: header issue.
Fixed proxymanager headers logic
Added proxymanager tests with tornado-based proxy
Fixed self.headers involuntary change
Improved readability a bit
Created dummyserver tests for ProxyManager
Added docs
Python 2.6 tests compatibility fixed
Tests are python 3.2 compatible now
Added old ProxyManager API compatibility