Switch API to using 'object_class' instead of 'queryset'.

Instantiating a queryset with every document in the database always
seemed odd.
Setting description and filename to string format

This prevents errors when importing PCAPs in STIX format. Previous error on import attempt was:

ValidationError(PCAP:None) (StringField only accepts string values:['description','filename'])
Merge pull request #160 from TheDr1ver/patch-2

Setting PCAP description and filename to string format
Merge branch 'master' into distributed_services