Feature sru

* added string import to PICA::Parser::XML (+ test)
* added Catmandu::Importer::SRU::Parser::picaxml
* added test 06-sru.t
* added test file sru_picaxml.xml (mockup)
* added Catmandu::SRU to cpanfile as prerequisite (**ATTENTION: Version 0.032 not on CPAN yet**)