Bug 2512: Initial design of YANG statement meta-model.

Bug 2512: Initial design of YANG statement meta-model.

Analysis of several new functional requirements for YANG parser,
showed  need to define meta-model of YANG model, which
could be used for transformation and processing of different YANG model
serialization formats (YANG text and YIN).

This new model introduces additional concept from YANG:

  - Statement
     - DeclaredStatement
     - EffectiveStatement
  - StatementDefinition
  - IdentifierNamespace

This basic building blocks allow us to describe with these APIs
any YANG statement model, which may be introduced down the line
in form of YANG extensions or updates to YANG specification,
which does not affect statement principle.

Change-Id: I9d42eaeba41fc388ed84dc7071d8e38755daab5d
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>
Signed-off-by: Robert Varga <rovarga@cisco.com>