Add support for TermVectorComponent

http://wiki.apache.org/solr/TermVectorComponent

Add support for term_vectors.

Example usage:
```
docs = si.query(phrase) \
		  .terms('speaking').terms(tf=True) \
		  .execute().result.docs
```

Currently assumes that the unique id is "id". I can clean up the xpath and add support for any unique id if you express interest in merging.

Thanks!