BUG-1201: make yangtools compilable with java 8.

Changed collection sorting in xtend classes.
Added maven profile to parent pom which is activated in java 8 environment to turn off DocLint.

Change-Id: Ief9dbaf9e029e653e337bf86740cdad0d07734f3
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-1201: make yangtools compilable with java 8."