Relax check for MARCXML so that the xml prolog is not required.
Merge pull request #270 from EreMaijala/marcxml-relax

Relax check for MARCXML so that the xml prolog is not required.
Simplified random Solr record retrieval.
Added support for ILS drivers interacting with non-Solr backends.
Reduced redundancy; added important missing fields.
More flexibility for request links (work in progress).
Add source to saved requests; cleaned up some quotes.
Implemented support for simple container-child record links using the hierarchy fields.
MultiBackend driver: Added second parameter to supportsMethod() calls, fixed some comments and tweaked variable names and debug messages.
MultiBackend: made getSourceFromParams more strict so that it doesn't try to get the source id from wrong fields and fixed the tests accordingly.