Added persistence module which can load which files have been 
processed from a standard log file format.
The AstromWorkload class now know what task it has been created for,
and handles locating the relevant files in the working directory.  Utility
for listing filenames with suffix moved to tasks module, with convenience
function for getting files for a specific task.
WIP: AstromWorkload accepts ProgressRecord on construction.
Filter out the files already processed for a specific task when constructing
the workload.
App now loads progress from log file.
Added ability for ProcessRecord object to write progress in addition to 
reading it.
Added file processed event which gets fired by the model when the 
sources corresponding to an astrom file are finished being processed.
ProgressRecord's flush method now immediately writes the data to
the file.  It will now also clear the file contents before re-writing them, to
avoid duplications.
The model now records progress when it completes processing a file in
the AstromWorkload.
ProcessRealsModel now opens MPC filehandle in append mode so that
when loading saved state you don't lose your old results.
Merge pull request #36 from drusk/save_filelevel

File-level saving