ENH: Lazily build the spatial index

Add a new property 'sindex' to GeoSeries and GeoDataFrame. External
functions should use the property to access the spatial index. The
rtree will not be built until the 'sindex' property is accessed.

A member '_sindex_built' is used to indicate if the rtree has been
built or needs to be.
Rename sindex caching method/members

_reset_sindex() -> _invalidate_sindex()
_sindex_built -> _sindex_valid
Merge pull request #158 from jwass/lazy_build_sindex

ENH: Don't build the spatial index until it's requested