Add index.html instead of FileUploader main class

It's slightly awkward having 2 main classes in the complete
project because ony one of them can run from the command line
by default. Consequently a user cannot take the "complete" project
and follow the instructions in the README because the server
cannot be run from there.

This change adds an index.html to the application so it
can be driven from a browser instead.

N.B. this sample is broken with Boot RC4 (RC3 still fine)
Merge branch 'feature/ui' of https://github.com/dsyer/gs-uploading-files into dsyer-feature/ui
Polish guide

* Make more updates to text of the guide to navigate with browser
* Restore maven-compiler-plugin version number
* Remove maven-exec-plugin since it's no longer needed
* Make path to /upload relative