lm4f: Add USB bulk device example

I wrote this example to benchmark the USB driver. See the README.md for
full details.

Why README.md instead of just README. README.md shows up better on github,
and makes it easier to browse the tree on github to search for an example.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
NOTFORGMERGE: Save some (unneeded?) examples

This is how my code looks before being cleaned up for merge. Remember kids,
don't try this at home!