added support for boosting fields and documents to python solr interface

Added the ability to set boosts on fields and documents when indexing. This is done through two new classes solr.Document and solr.Field
c = solr.SolrConnection(host='localhost:8081')
c.add(id='123', name=solr.Field('this is a field', boost=1.5))
doc = solr.Document(boost=1.5)
doc.add(solr.Field(name='title', value="a value for my field", boost=1.1))
c.addDoc(doc)