Changed EnsembleArray neurons parameter to be per-ensemble (Issue #242)

As discussed, now when creating an EnsembleArray, you specify the neurons for one ensemble and then those get duplicated in each element in the array, rather than specifying the total neurons and then splitting them up.  Also note that as part of this I removed the `dimensions_per_ensemble` parameter.  Since all the parameters are per-ensemble now, you just specify `dimension`, like you would for a single ensemble (or if you don't specify anything the default is 1).

Before:
```python
EnsembleArray(nengo.LIF(N*n_ensembles), n_ensembles, dimensions_per_ensemble=2)
```

Now:
```python
EnsembleArray(nengo.LIF(N), n_ensembles, dimensions=2)
```