Add S4 class xapian_database

* R/S4_classes.r: Add S4 class xapian_database to path(s) to Xapian
  databases.

* R/database.r: Add methods show and summary for a S4 class
  xapian_database.

* DESCRIPTION: Add 'S4_classes.r' and 'database.r' to Collate field.

* Update NAMESPACE
Import plyr package
Improvements to 'xsearch' method

* Add view argument to invoke a text editor on the result before
  returning. This enables editing the result before it's returned. The
  editor is only opened if there are any response variables supplied
  in the prefix formula.

* Use rbind.fill to combine data.frames and handle missing columns.
Change 'xindex' method to return S4 class xapian_database
Update documentation
Import methods