Check for xdrop extensions of length 0

came up in #291
Merge pull request #292 from Garonenur/xdrop_bugs

Check for xdrop extensions of length 0
add getter fkt and tests
Closes #293.
Merge pull request #295 from standage/master

Closes #293.
Merge pull request #294 from Garonenur/intset

add getter fkt and tests to intset classes
fix typos
unify method names
gtlua: fix sequence extraction
gtlua: fix gt.translate_dna()
gtlua: add meta node accessors
add dev tool for spec checker
gtlua: add 'genome_node:get_line_number()'
add GtSpecResults
add gt_comment_node_(try_)cast()
add spec results support
add detailed spec check reporting
src_clean
gtlua: add comment node methods
add spec checker output for more node types