Check for null proxies in ~CmdCallbackI
Rationalize Fixture handling in OmeroCpp tests

The handling of the client objects was unwieldy. Now
across the board Fixtures have access to a local root
instance regardless of login, and a single login is
activated on demand. The destructor of all Fixtures
cleanup any non-null omero::client_ptr objects. This
is intended to workaround any "too many open file"
issues.

NB: Also removes tabs on any change file.
Merge pull request #752 from JesseCorrington/cpp-fixture-develop

Integrate cpp fixture changes into develop