EventManager generates uuid for each simulation that can by used as desired by
backends. SqliteBack now generates a short int id associated with the sim
uuid and tags every row entry with the short sim id.
fixed comments, cleaned includes, fixed SqliteBack regression test
addressed many of gidden's suggestions.  Removed SqliteBack's dependency on singleton EventManager and added multisim test.
Separated the abstract base class EventBackend from the header file of the EventManager. Now Backends can be instantiated without also instatiating the EventManager singleton.
Merge pull request #481 from rwcarlsen/multisim

support for multiple simulations in single output db