Added support for parsing yang models with already resolved context.

Added support for parsing yang models with already resolved context.

Added rebuild method to AugmentationTargetBuilder interface to support augmentation of resolved context.
Refactored YangParserImpl to improve readability, internal utility methods moved to ParserUtils class.
Added test to check parsing with context.
Added YangErrorListener to redirect antlr4 error messages from standard error output to logger.
Added more javadocs.

Change-Id: I838646cc4b3593a03420c1b578ee39c043f89e3d
Signed-off-by: Martin Vitez <mvitez@cisco.com>