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.
Shouldn't we have something similar to a `GetResult` for term vectors? In this case it'd require a bit more work, and all of it should go to a different PR?