added a sub_Slice method, that returns a smaller slice. This will not make use of the cached features of the parent slice though

just gene, transcript, and exon table;

to enable lazy loading

removed warning for shared exons

Made check for insertion failure stricter. sth->execute could return "0E0" which perl evaluates as true but zero (fix from lec)

fix: briggsae contig<->clone assembly table info was incorrect because embl_offset was not set in source database

should only have thrown if one could not be retrieved from current db

* made store method safe for multiple processes doing simultaneous storing by using INSERT IGNORE and getting count of actually stored
* also removed dbh->do statements and replaced with dbh->prepare