Add method to search a Xapian database

* R/xapr_search.r (xapr_search): Add R method to search a Xapian
  database given a path and search terms. Calls internal cpp function
  and returns list with result.

* src/xapr.cpp (xapr_search): Add cpp function to open and search a
  Xapian database.

* src/Makevars: Link to xapian