BUG-580: Improved parsing.

Added new YangContextParser interface.
Deprecated methods from YangModelParser.
Improved IO and exception handling.

Change-Id: I2a07320acab3d19570b0ddb645b5113a531a8532
Signed-off-by: Martin Vitez <mvitez@cisco.com>
Merge "BUG-580: Improved parsing."