Allow login to POST to custom challenger obtained from form

The ts.js has an options literal that can have a custom challenger set against it but this option is never utilised.

This commit allows the login code to obtain a custom challenger to use for login from the form by reading a hidden field value.  If it is not provided it will use the default tiddlyspace cookie form challenger.

This commit also reports on test coverage achieved so far.
Correct whitespace
Properly fixing whitespace
Set custom challenger as action based on @cdent's suggestion
Ensure ajax method is always restored in case of a test failure
Merge pull request #12 from TiddlySpace/customchallenger

Allow Login to POST to Custom Challenger Obtained from Form