[bugfix] Splitted addModel into adding and populating

SNIp only had a "addModel" method that was used to add a model
and populate the Node associated to the model. Now the two
operations are splitted. The model will only call addModel when
the socialNetwork is set. Populate is called when SNMI::populate()
or SNMI::repopulate() is called.

This commit also fix a bug that might happen, when, when the user
uses the same model to load a different node, the old node
was not purged.
[feature] Adding Twitter support

This commit initiates Twitter support. It imports some important
classes, that are related to request signing. It also relayout
tools and tests, and prepare the tools to generate classes for
any social network.

This commit also includes a Twitter test that should be able to
perform PIN based authentification.
Initial work on TwitterInterface

This commit includes some initial work on TwitterInterface
to provide a Twitter specific subclass of
SocialNetworkInterface.

It provides initial support of TwitterUserInterface, and
includes the capability to query followers and followe.
Added tweet support