Term Vectors: Support for artificial documents

This adds the ability to the Term Vector API to generate term vectors for
artifical documents, that is for documents not present in the index. Following
a similar syntax to the Percolator API, a new 'doc' parameter is used, instead
of '_id', that specifies the document of interest. The parameters '_index' and
'_type' determine the mapping and therefore analyzers to apply to each value
field.
Yep! It was just cut and paste of [that](https://github.com/apache/lucene-solr/blob/trunk/lucene/core/src/java/org/apache/lucene/document/Document.java#L236), but I agree.