Merge pull request #29 from standage/master

Change behavior or gt_feature_node_set_source to enable resetting source value
refactor and rename

removed indirection for reading and writing were possible
Merge pull request #30 from Garonenur/dw

refactor and rename
change Sascha Kastens' mail address
Merge pull request #31 from satta/master

change Sascha Kastens' mail address