[Fix #174] Ignore events targeting other windows

 * define targetIsThisWindow, which compares the target attribute of
   <a>s and <form>s
 * use targetIsThisWindow to filter 'click.history-*' events on <a>s
   and 'submit' events on <form>s
Expose Sammy.targetIsThisWindow

Change the private helper function to one on the Sammy object so clients
can override it if they need to.
Merge pull request #179 from jamesarosen/fix_174

Fix #174 - better "target" checking