remove obsolete header file
restore old semantic for gt_meta_node_cast()

and simplify code
add gt_region_node_cast() to API

with old semantic
restore old semantic for gt_sequence_node_cast()

and simplify code
add gt_feature_node_cast() to API

with old semantic
update API documentation
Merge pull request #58 from gordon/master

Document undocumented API method
Merge pull request #59 from gordon/cast

restore the old cast semantic