* single file per rank output creates one file per rank
* global file output creates one file only from rank 0

* Address issue #74: missing json configuration options.
* Allow parameters from input file to be overriden on command line.
* Missing parameters in json input will defer to defaults.
* Add option for saving current options to json file (allowing
  for json parameter file format discovery.)
* Fix annoying command line argument reordering from TCLAP.
* Fix bug in assignment of `nothing` to a util::optional<T>
  variable.

Default is false
No output via command line parameters

New options:
 * -p, --probe-ratio      Attach probes to this proportion of cells
 * -X, --probe-soma-only  Don't probe dendrites
 * -P, --trace-prefix     Specify where to write traces
                          (default: 'trace_')
 * -T, --trace-max-gid    Don't trace cells with a larger gid than this.

* -S (--syntype) option passes argument through to
  synapse creation in `make_cell`.

- improve robustness of time step and time integration
  interval selection
- changes of the type `if (!id)` to `if (id==0)`.
- fix bug selecting source cell gid when generating
  network connections
- make all-to-all network a user-specified option
    - updated README
- fix bug where an initial spike was not generated for
  the first cell on a communicator when the cell gid
  was a multiple of 20

* the user can now specify simulation time and time step size
    - via command line arguments
    - via json input file

* add header only library TCLAP for parsing command line arguments
    - added as git submodule
    - update CMakeLists to include headers
* remove the ring model from miniapp
* add support for user supplied parameters for miniapp
    - arguments for cell counts etc can be command line flags
    - or in a user-defined json input file
* the all-to-all model supports synapses-per-cell is less than
  the total number of cells
    - in this case a random set of sources is selected