Deprecate GtFeatureStream, replace with GtFeatureInStream

Addresses comments raised in #56. Simply doing a typedef of `gt_feature_in_stream_new` to `gt_feature_stream_new` gave the desired functionality, but did not generate the desired (HTML) documentation. Thus the trivial constructor implementation.

As far as region node ranges, the default functionality is probably sufficient. I'll worry about providing the option for `gt_feature_index_get_orig_range_for_seqid` if the need ever arises.