change FacebookSession class variable visibility to protected

[inoryy]

Simple change to allow extension of this class.

Specifically, trying to override constructor method is impossible because of private visibility.

[weaverryan]

This relates to #79 and would allow us to sub-class and not start the session only in the test environment. We've all got this file hot-patched locally when we run our tests, so this should make life easier :).

So, +1 for my end

[stof]

I merged #161 instead as it was sent first with the same change