Add initial stub of GerritClient class
Add initial stub of GerritClient class

The Gerrit client class will be used as an interface to the
Gerrit events stream and query functionality.

This is the intial stub.  More functionality will be added,
and existing stream functionality refactored, in later
commits.

Change-Id: If4ef838c2d3f3e5afaad2a553af49b1c66ad4278

dpursehouse/pygerrit2,markon/pygerrit2,dpursehouse/pygerrit,benjiii/pygerrit,gferon/pygerrit2,morucci/pygerrit,sonyxperiadev/pygerrit