Gridmap

- Fixed a bug where only the first error was still showing because of an extra exception caused by job_output being undefined.
- Fixed unhandled Exception with error code 24 (since somehow that is not an InvalidJobException, but just an Exception in drmaa-python).

- No longer dies with InvalidJobException when failing to retrieve job metadata from DRMAA service.
- Now print all exceptions encountered for jobs submitted instead of just exiting after first one.
- Die via exception instead of sys.exit when there were problems with some of the submitted jobs.

- Fixed bug where jobs were being aborted before they ran.

- Fixed bug where GRID_MAP_USE_MEM_FREE would only be interpretted as true if spelled 'True'.
- Added documentation describing how to override constants.

- Added support for overriding the default queue and other constants via environment variables. For example, to change the default queue, just set the environment variable GRID_MAP_DEFAULT_QUEUE.
- Substantially more information is given about crashing jobs when we fail to unpickle the results from the Redis database.

- Fixed serious bug where gridmap could not be imported in some instances.
- Refactored things a bit so there is no longer one large module with all of the code in it. (Doesn't change package interface)