Force m2e to execute most goals (non-incrementally)

Eclipse's Maven integration requires certain extra steps of each Maven
plugin, to state how compatible the plugin is with Eclipse's version of
a lifecycle. With this change, we tell Eclipse to execute most NAR
goals, but only when building from scratch (because the steps are pretty
expensive).

Some of the goals are also triggered upon configuration, e.g. the
generation of a NarSystem class or downloading/unpacking dependencies.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
eclipse style fixed.
Let Eclipse know about generated NarSystem classes

According to http://wiki.eclipse.org/M2E_compatible_maven_plugins we
should use the BuildContext to inform Eclipse about updated and created
source files.

Happily, the BuildContext allows us to do that without having to link to
Eclipse proper, let alone having to write and deploy a full-blown Eclipse
plugin.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge pull request #130 from maven-nar/eclipse

Improve Eclipse support