Update httpclient dependency in driver module

[simonedavico]

HttpTransport in Faban's driver module currently leverages an old implementation of Apache's httpclient library (3.1).

Since we have to use HttpTransport to route calls to the SUT endpoints, at some point we should update HttpTransport (and all related classes) to leverage the latest version of the httpclient library.

This will not only provide us a more efficient version of the library, but will also give us the possibility to abstract HttpTransport in our implementation, providing a more user friendly API.

[VincenzoFerme]

References for performance difference 3.1 vs latest version:

• http://blog.kennardconsulting.com/2011/01/httpclient-4-want-to-make-it-250x.html
• http://stackoverflow.com/questions/21644588/difference-in-response-time-between-httpclient-3-1-and-http-client-4-in-jmeter
• http://markmail.org/message/ox7dz3uaovtbqwkt

This only impacts the velocity by which a new connection is started, not the computed metrics.