Add coordinate indexing

This is a new API for slicing `GeoSeries` objects by coordinates.  This implements a new indexer that uses the `cx` attribute for slicing by coordinates, analogous to `ix` for slicing by index.  Slices on `cx` will return all geometries that intersect the bounding box defined by the sliced coordinates.

Examples:

    >>> g = GeoSeries([Point(1.0, 1.0), Point(2.0, 1.0), Point(1.0, 2.0), Point(2.0, 2.0)])
    >>> g
    0    POINT (1.0000000000000000 1.0000000000000000)
    1    POINT (2.0000000000000000 1.0000000000000000)
    2    POINT (1.0000000000000000 2.0000000000000000)
    3    POINT (2.0000000000000000 2.0000000000000000)
    dtype: object
    >>> g.cx[:, :1.0]
    0    POINT (1.0000000000000000 1.0000000000000000)
    1    POINT (2.0000000000000000 1.0000000000000000)
    dtype: object
    >>> g.cx[2:, 2:]
    3    POINT (2.0000000000000000 2.0000000000000000)

A `step` (stride) is not handled, since coordinate intervals are continuous.

This is a preliminary implementation, open for discussion on the interface.