Added Gateway interface

The Gateway interface facilitates convenience classes that want to
provide one-line service instantiation through a wrapped Context. The
provided get() methods allow simple and consistent Service lookup, with
a NoSuchServiceException thrown if the requested Service is not found.
Added Gateway example

The Gateway Javadoc now provides a sample implementation to demonstrate
its use and benefits.
Merge branch 'gateway'

This provides a Gateway class hierarchy for use with convenience layers
such as imagej.ImageJ and ome.scifio.SCIFIO.

See the Gateway javadoc for details.
release-version.sh: verify that work tree is clean

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
release-version.sh: deploy release version

This builds and deploys the tagged release to the Maven repository.

Dictated-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Bump to next development cycle

Signed-off-by: Curtis Rueden <ctrueden@wisc.edu>