Logging

This pull request is mostly a request for review and comments. 

I have added a new (central) logging toolbox that does not have any external dependencies (no ACE) and so it should be usable in toolboxes for standalone apps as well as in Gadgets. It is basically a singleton which uses an environment variable (GADGETRON_LOG_MASK) to set up which log levels are printed and whether file locations should be printed as well. The singleton is access with some simple macros GDEBUG, GINFO, GWARN, GERROR. 

It gives us some runtime control over what is logged and solves the ACE dependency problem. 

It uses a printf sort of syntax. The std::cout syntax introduced by Hui is really bad in multithreaded programming and it would take some tricks to make that work well.

Let me know what you think.