Bug 499: Initial implementation of data tree modifications

Initial implementation of data tree modifications using
StoreMetadataNode and NodeModification.

  - DataAndMetadataSnapshot - helper class, which
      contains immutable snapshot of data and metadata

  - ModificationApplyOperation - interface definition
      of composite hierarchical operation, which
      is responsible for applying changes to provided
      subtree

  - SchemaAwareApplyOperation - implementations of
      ModificationApplyOperation based on parsed
      SchemaContext

  - MutableDataTree - class which integrates
     Data tree, Metadata tree, Node Modification tree
     and ModificationApplyOperations into mutable logical
     tree, which will serves as a basis for read-write
     transactions.

  - Unit test for MutableDataTree in following scenarios
      - Create empty MutableDataTree, write subtree, read nested nodes
      - Create empty MutableDataTree, write subtree, modify subtree
      - Create MutableDataTree with existing date, write changes and deletes

Change-Id: I5c0c84764f93d150eac6d227c4bc367b67652f9d
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>