Added Support for Union Type def resolving and bug fixes.

Added implementation into TypeProviderImpl to proper resolve union type definitions;
Added UnionDependencySort for correct resolution of multiple unions in same yang model;
Added support for resolving of imported union types from multiple modules;
Added tests for Union Type def resolving;

Fixed Resolving of typedef Enumerations - now the Enumeration is resolved as standalone enum typedef name .java;
Fixed resolving of Inner Enumerations;
Fixed resolving of referenced Inner Enumerations through leafref definition;
Added EnumGenerator class for generating of Enum definitions and inner Enum definitions;
Fixed generation of Enumeration type through code writer;
Fixed generation of Enumeration package name in case of inner enum definition in Class and Generated Transfer Objects;
Added capability to generate inner enums into ClassCodeGenerator and InterfaceGenerator;

Fixed of missing apostrophe in createToString method in GeneratorUtil.java in case when more than one property should be generated in toString method;

Fixed platform specific bug in maven-yang-plugin incorrect retieval of maven dependency in YangToSourcesProcessor.java;
Fixed platform specific bug in maven-yang-plugin-it - hard coded resource paths replaced for getClass().getResource calls;

Added support into BindingGenerator API to suppor of generation of Types with restriction of specified modules that will be generated;
Added implementation of generateTypes in BindingGeneratorImpl;

Added tests and test resources for testing Enumeration gen. types
Added test resources for augment relative xpath testing;

Fixed improper implementation of hashCode and equals in ExtendedType in yang-model-util;
Fixed findParentModule method in SchemaContextUtil in yang-model-util project;

Change-Id: I144acbeec9ce547da856e2979d27b2c072174a52
Signed-off-by: Lukas Sedlak <lsedlak@cisco.com>