Fix for 32 vs 64-bit integer ambiguity

[sgillies]

This issue was reported in geopandas/geopandas#691 and probably also complicates PR #540 in some way.

@snorfalorpagus I think it's time to make 'int' in a Fiona schema mean 64-bit integer as in Python and Numpy.

>>> numpy.dtype('int')
dtype('int64')

and, instead of relying on integer field width, add a new 'int32' name here: https://github.com/Toblerity/Fiona/blob/master/fiona/ogrext.pyx#L48 for 32 bit-integers.

Fiona 1.8 will then fix that particular GeoPandas issue. We will not backport this to 1.7, where there is a field width workaround: int:18 for big ints.