Perseids treebank

Changes to support treebanking and review functionality, including some general improvements to tool configuration:

* tool configuration in tools.yml now supports id_param and replace_param settings - id_param will be replaced automatically by the identifer id(s) of the identifier being processed, replace_param can be used to support a 2nd multi-valued parameter in the tool url to be replaced by some value from sosol controller
* new Compare link on the edit tool bar is present for treebank and alignment cite identifiers if the @identifier var created in the Identifier controller contains a :compare key/value pair.  
* many changes and improvements to the treebank_cite_identifier functionality for calculating matches between two different identifiers based upon the values in the document_id and subdoc attributes on the sentences
* using Nokogiri for SAX parsing of attributes in treebank_cite_identifier xml documents for cases when a full DOM parse isn't needed (such as in comparing attribute contents for matching files). Improves performance of create and save requests. 
* Created anew xml_helper module which can be used to abstract xml handling from within an identifier to different parsers. Used only in treebank_cite_identifier.rb so far, for Sax Parsing per previous item.  Apparent bugs in Nokogiri behavior for DOM parsing prevented full replacement of use of REXML for this identifier class though.