- instrument() and most wrappers gain an overwrite argument to allow the user 
   to ensure that an instrument is not overwritten.
 - load.instruments() gains an overwrite argument that is passed through to the
   instrument() wrapper functions.  Therefore, load.instruments() no longer 
   checks to see if an instrument already exists.  Thanks to Charlie Friedemann
   for the patch.
 - load.instruments() gains an "identifier_cols" argument that can be used to
   pass the names of columns that are to be used as identifiers instead of as
   normal instrument attributes.  Thanks to Charlie Friedemann for the patch
 - minor code formatting; tabs to spaces, etc.



git-svn-id: svn://svn.r-forge.r-project.org/svnroot/blotter/pkg/FinancialInstrument@1473 edb9625f-4e0d-4859-8d74-9fd3b1da38cb