Add a executor to https with client certificate
Using only urllib2_executer configurable
Make urllib2_executor accepts handlers

* use() accepts HTTPHandlers
* Remove HttpsHandler and its ports
Merge pull request #64 from gisce/add_https_client_cert_executor

Add support for adding custom handlers to the urllib2 executor.
executors/urllib2: use a single argument for extra handlers

This will allow adding new arguments to urllib2 executor's use() method in the
future, if need arises.
executors/urllib2: use camel case for class name