Fixes #14 Properly checks dimensional lengths using ndim.

Auxiliary coordinate variables are also allowed, so the dim==name portion has been removed.  Reformatted check to properly check for the increase in dimensionality.
Since the return value of `_find_boundary_vars` is a dict mapping coord var to associated boundary var, you could make this more concise:

```
for cvar, bvar in self._find_boundary_vars(ds).iteritems():
    if bvar.ndim == cvar.ndim + 1: