expose the sinatra `send_file` method through to the view handler

This exposes sinatra's `send_file` method to deas view handlers
for file sending goodness.

Includes a test runner variant that returns the named args for
testing goodness.

Nothing new here - just piping stuff through like all the other
protected view handler sinatra methods.
Merge pull request #80 from redding/kr-sendfile

expose the sinatra `send_file` method through to the view handler