Add console service

This general-purpose, non-image-specific service migrated from ImageJ:
https://github.com/imagej/imagej/tree/imagej-2.0.0-beta-7.8/core/core/src/main/java/imagej/console
Add I/O service

This general-purpose, non-image-specific service migrated from ImageJ:
https://github.com/imagej/imagej/tree/imagej-2.0.0-beta-7.8/core/core/src/main/java/imagej/io
IOEvent: tweak javadoc

Opening and saving are two possible I/O types, but not necessarily the
*only* possible types.
IOEvent: also report data for which I/O happened
IOService: fire event when data is opened or saved
Add text service

This general-purpose, non-image-specific service migrated from ImageJ:
https://github.com/imagej/imagej/tree/imagej-2.0.0-beta-7.8/core/core/src/main/java/imagej/text
Merge pull request #39 from scijava/new-services

Migrate some services from ImageJ