API Claratity: Documented package o.o.y.yang.model.util

Documented package o.o.y.yang.model.util,
documented differences between base YANG types
which resulted in different instantiation pattern for
some of built-in types.

Deprecated utility YangTypesConverter which name was
confusing, added replacement methods in BaseTypes
which name is less confusing and more clearly
comunicates its functions.

Improved hashCode and equals after introducing
static constants.

Change-Id: I300d64cb38ded6989afa0eb841dca61a0efbf8f0
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>