improve xdrop

add test for backward xdrop for perfect matches
adjust comment
fix positions

xdrop positions in ltrharvest_stream corrected
remove zero length xdrop

do not call xdrop with zero length

add assertion to GtSeqabstract to prevent zero length sequences
positions and assertions

there is the possibility for GtSeqabstracts to have zero length now.
I fixed every occurence where they are used with xdrop, but some code
uses GtSeqabstracts not with xdrop. There zero length GtSeqabstracts
happen.
Merge pull request #240 from Garonenur/GtSeqabstract

Gt seqabstract
fix node order for sequence/EOF nodes
fix failing Ruby tests on 64-bit Linux
remove debug output
Merge pull request #243 from satta/node_order

fix node order for sequence/EOF nodes
fix compilation on Mac OS X (10.8.5)

tre library code triggers self-assignment warning, patched tre-compile.c
Merge pull request #244 from satta/ruby64

fix failing Ruby tests on 64-bit Linux
fix failing gttestdata tests

these changes require a new gttestdata as well, please pull.
Merge pull request #247 from satta/mac_compile_warn

fix compilation on Mac OS X (10.8.5)
Merge pull request #248 from satta/gttestdatafails

fix failing gttestdata tests
Closes #249.
Merge branch 'fis' of github.com:standage/genometools into standage-fis
coding convention