Add feature to handle server errors after the user submits

[micahcourey]

This feature is meant to solve issue #6 "How to handle error case on Done?". I added a revertToStep method which sets _isComplete back to false and then returns the user back to the specified step. The revertToStep method will is fired by adding forceStep and the step number to the error handler portion of your post method. Here is an example:

postForm() { ... }, (error) => { this.forceStep = 1; } }