Adding an argument to the adapter for passing a block argument to the connection pool.

This allows for blocking when using threading to prevent the pool from creating more connections that the max-size allows.

Specifically was seeing the following errors without the block=True:

    WARNING:requests.packages.urllib3.connectionpool:HttpConnectionPool is full, discarding connection: www.example.com
Moving the order of the arguments in the init to not interfere with existing usage.
Fixing the call to init_poolmanagers to correctly unpickle the adapter.
Adding the _pool_bloc to the list of attrs.
Merge pull request #1356 from Zoramite/ConnectionPoolArgs

Adding an argument to the adapter for passing a block argument