added headers to document

[bramj]

• document now has access to @headers, a hash in format :header_file_path => :nokogiri_object
• search for document bookmarks also includes bookmarks in headers
• headers are also saved upon document save
• some tests for this stuff

[armilam]

Man, I wish this was merged. I am going to try using this code in my software to extract a doc header. Hopefully we could get this functionality added to the gem!