Handle array-like parameters for native synapses

* Array-like parameters are converted to Sequences for pyNN.
* Sequences are converted back to numpy-arrays when calling the
  nest-kernel.
PyNN.nest: Projections deal with CommonProperties

* Also added testcase.
pyNN.nest: Further handling of syn params

* ValueError is raised when CommonProperties of synapse type is assigned
  different values in the same Projection (validated by added testcase).

* Shape (1,) arrays of parameters (as produced by the FromListConnector)
  are converted into the corresponding scalar because otherwise
  nest.SetDefaults interprets them as vectors.

* numpy.bool_ are converted to bool because nest.SetDefaults does not
  handle them.
pyNN.nest: Clarified error message

* NEST seemingly has no way of removing wrongfully created connections.
  Hence the user is instructed to rebuild his network when erroneously
  using heterogeneous common synapse properties.
Fixed proper ndarrays not being converted
Further pynn/sli compatability fixes

* Streamlined identification of common synapse properties in
  Projections. Once a _convergent_connect is called, a list
  self._common_synapse_property_names is available (this is used in
  _set_attributes and shall be used in _get_attributes_xy in the
  future).

* Projection._set_attributes now checks for common synapse properties
  and makes parameters sli compatible prior to invoking pynest.

* native_cell_type now also makes its default parameters pynn
  compatible. The backtransformation (of Sequences to numpy arrays) is
  already handled by _build_params.
Misplaced bracket in comment
Added section about common synapse properties to doc

* Also modified AUTHORS.
Merge pull request #283 from obreitwi/nest_fixes

pyNN.nest: Fixed being unable to use native synapse types that used either array-like parameters or common properties.