Bug fixes and improvements

Fixed command line options (no duplicates, correct escaping and using single-dash instead of double-dash as the latest jdeps requires)
Fixed java toolchain support
Bound to verify phase by default for bound executions
Fixed logging
Bound mojo parameters to system properties that can be specified on command line
This activates support for Maven toolchains in building the plugin (I must admit I have become am a big fan of those). The main advantage is that all java tools (compiler, surefire, javadoc,...) are consistently applied using the same selected JDK, You will need to knock up ~/.m2/toolchains.xml to specify where java 1.7 sits in your file system.

Now for the reasoning:
The plugin itself does not require anything from java1.8. Invoking jdeps is a separate step from running the plugin as part of Maven build. Most of the Maven builds I came across recently are still running under java 1.7, and most of the existing core Maven plugins are still compatible all the way back to java1.5. I think it would be unreasonable to force people to run Maven under java1.8 just because one plugin requires it, when you can just as well run Maven under java1.7 and select java1.8 in a toolchain for forked jdeps executions.
Does it make sense?