Fix for bug in GTF get_attribute_by_name
Created by: davidrichardson
The method get_attribute_by_name in modules/Bio/EnsEMBL/IO/Parser/GTF.pm fails for ensembl GTF files, due to a space between ; and the attribute key. Test case for GTF files is included.
Also removed the last use of throw from modules/Bio/EnsEMBL/IO/TextParser.pm