boost factor

Related to issue #74

I have introduced a new parameter for LuceneDatabase: boost-factor. The parameter value can be one of the following: 

QUERY (default) : the boost factor is used at query time (no need to re-index if you want to change the weights)
INDEX:  the boost factor is used at index time (slightly faster than QUERY but less flexible)
NONE: boost factor is not used at all

I have done some test trying to LINK 100K entities from Source 1 and 3M entities from Source 2.
I have not seen any relevant improvement in term of performance but I have a better recall (more links found).