Merge pull request #51 from gordon/master

website update
Made gt_feature_node_try_cast part of public API
Minor improvements to docs and code for 'gt_feature_node_try_cast'
Made gt_region_node_try_cast part of public API
Made gt_sequence_node_try_cast part of public API
Made gt_eof_node_try_cast part of public API
Made gt_meta_node_try_cast part of public API
Implemented GtFeatureOutStream class
Merge pull request #54 from standage/try-cast-api

Made gt_feature_node_try_cast part of public API
Merge pull request #57 from standage/feature-out-stream

Implemented GtFeatureOutStream class
document API method and add assertion
update library documentation