Added skeleton for yang maven plugin.

Added skeleton for yang maven plugin.

Added implementation of yang-to-sources maven plugin
which is responsible for parsing YANG files and invoking
configured code providers to generate sources from YANG
during maven generate-sources and generate-resources phase.

Added JUNIT tests testing the integration with maven and
following scenarios:

No generators for generate-sources and generate-resources
Unknown generators for generate-sources and generate-resources
Non-existant root folder with yang sources
No yang sources under yang root folder
Correct configuration for both generate-sources and generate-resources

Change-Id: Ib4d2a2c004ba19d3fda79d6a3d6da41050059ce0
Signed-off-by: Maros Marsalek <mmarsale@cisco.com>