Cleanup: unused code, minor simplifications, wrap lines and comments.
Cleanup: use more verbose identifier.
Add a fixme-comment.
Cleanup: Remove implementation of class HeaderAwareHTTPDownloader.

The code is the same as twisted.web.client.HTTPDownloader since at
least twisted 1.3.0.
Cleanup: Remove implementation of class myHTTPPageGetter.

The code is the same as twisted.web.client.HTTPDownloader since at
least twisted 8.0.0.
Fix typo.
Cleanup class ProxyClient (part 1): Remove method implementations.

The code of the removed methods is the same as in twisted.web since long.
Cleanup class ProxyClient (part 2): call super-class methods.

Instead of re-implementing the super-class methods call/inherit them.
Cleanup: Remove implementation of class ProxyClientFactory.

The code of the removed methods is the same as in twisted.web since long.
Add a fixme-comment.
Cleanup: Implement downloadPage like twisted.web.client.downloadPage.

This also fixes github issue #3.
Add a comment about the differneces between out implementation ...

... and the one in twisted.web.client.