Bug 1436: Introduced BindingStreamEventWriter interface

Introduced BindingStreamEventWriter interface,
which allows to implement Binding codecs
with plugable backend for actual serialization.

This interface needs to present in yang.binding
package, since this in only package which
is imported by all generated YANG Java Bindings
and thus visible for them.

Change-Id: I2d235cd63ecda1d63c8077854a38a5dc9d184d2d
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>
Merge "Bug 1436: Introduced BindingStreamEventWriter interface"