Add the 'using-ops' tutorial

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add the 'create-a-new-op' tutorial

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
UsingOps: use simpler lookup for add op

The "out" argument is optional, so we leave it off.
UsingOps: flesh out the usage examples
UsingOps: add count of available ops
CreatingANewOp: spice up the op a little

We also split the execution from the later print statement.
Rename create-a-new-op main class, for consistency
CreateANewOp: tweak javadoc
POM: update pom-scijava parent to 1.155