gtdoc: allow more optional words (returns( in API functions
emphasize GenomeTools in API documentation (unified style)
extend documentation of GtTool class
document gt_xfwrite_one() and gt_xfread_one() properly
improve documentation of timer API
improve documentation of codon iterator API
rename constructor (naming convenstions & fixing API doc)
docu: define reverse module
enforce naming conventions and improve documentation

gt_output_file_register_options() -> gt_output_file_info_register_options()
fix minor uninitialized values etc.