Fixed all of the tests/bugs.  Test error output improvements. Minor API change.

This includes changes submitted in pull request #1.  My additions:

* assertSeqEqual was broken up into 3 type-safe, simpler alternatives that use the more informative testing.Fatal methods.
* failing Diff assertions now print the line of the actual problem along with some pretty diff info.
* fixed all failing tests
* added a New method to replace the private createDMP to make the package a bit nicer to use and more Go idiomatic.
* DiffMain now is a wrapper around diffMain and takes no interface{} args, and 1 bool (checklines) arg (only backwards-incompatible change).
* other misc tweaks.