Fix Beanshell example in the README

For consistency, we will call the gateway function op(), not
ops(). Also, the map() function needs to be called on the OpService,
not the gateway.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add two ops necessary for the using-ops tutorial

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add operation to dump ASCII version of an image

Useful for debugging, and demonstration purposes.
DefaultASCII: allow explicit passing of min & max
Merge branch 'readme'

This branch updates the README to make the Getting Started examples more
correct.
README.md: use more whitespace in BSH example

I found the example pretty hard to read before. The space does make the
example longer, but also easier on the eyes IMHO.