Add [Field(Boost = float)] and [Field(TermVector = TermVectorMode)] attribute properties and TermFreqVectorDocumentMapper.

Fixes #12.
Work around Lucene/Lucene.Net known issue when applying Boost to NumericField at index time.

See https://issues.apache.org/jira/browse/LUCENENET-519
Update release notes.
Add .gitattributes using Windows (CRLF) line endings for text files.
Add travis-cs configuration.