Separated out proxy_manager_for to fix #2048

Basically identical to @sigmavirus24's suggested code - thanks for that pointer. Just added docstring.

There aren't tests for particular Adapter implementations, and I've not added any at this stage.
I have tested it with the following (which achieves what I want):

    class SourceAddressAdapter(requests.adapters.HTTPAdapter):
        """
        A Requests HTTP adapter to add source address specification for proxies
        """
        def __init__(self, *o, **k):
            self.source_address = k.pop('source_address', None)
            super(SourceAddressAdapter, self).__init__(*o, **k)

        def proxy_manager_for(self, proxy):
            conn_params = {}
            if self.source_address is not None:
                conn_params['source_address'] = (self.source_address, 0)

            if not proxy in self.proxy_manager:
                proxy_headers = self.proxy_headers(proxy)
                self.proxy_manager[proxy] = requests.adapters.proxy_from_url(
                    proxy,
                    proxy_headers=proxy_headers,
                    num_pools=self._pool_connections,
                    maxsize=self._pool_maxsize,
                    block=self._pool_block,
                    **conn_params)

            return self.proxy_manager[proxy]
Kenneth is going to hate this indentation, can we fix it up? Should look like this:

```python
self.proxy_manager[proxy] = proxy_from_url(
    proxy,
    proxy_headers=proxy_headers,
    num_pools=self._pool_connections,
    maxsize=self._pool_maxsize,
    block=self._pool_block,
    **proxy_kwargs
)
```