Add suport for anydata and anyxml statements

[lgifre]

Hi Rob,

While generating bindings for a model in RFC 8040 Section 8 (https://tools.ietf.org/html/rfc8040#section-8), pyangbind is reporting an exception that seem to be a bug. The model (simplified by removing non-relevant parts) is listed below. The same exception is raised when you use anyxml instead of anydata.

module model {
	yang-version 1.1;
	namespace "test:model";
	prefix "m";

	container error {
		anydata error-info;
		//anyxml error-info;
	}
}

I'm generating the bindings using:

export PYBINDPLUGIN=`/usr/bin/env python -c 'import pyangbind; import os; print "%s/plugin" % os.path.dirname(pyangbind.__file__)'`
pyang --plugindir /usr/local/lib/python2.7/dist-packages/pyangbind/plugin -f pybind -o bindings.py model.yang

And pyangbind raises the next exception. Pyangbind raises the same exception when your model contains anyxml instead of anydata:

Traceback (most recent call last):
  File "/usr/local/bin/pyang", line 444, in <module>
    run()
  File "/usr/local/bin/pyang", line 413, in run
    emit_obj.emit(ctx, modules, fd)
  File "/usr/local/lib/python2.7/dist-packages/pyangbind/plugin/pybind.py", line 224, in emit
    build_pybind(ctx, modules, fd)
  File "/usr/local/lib/python2.7/dist-packages/pyangbind/plugin/pybind.py", line 426, in build_pybind
    get_children(ctx, fd, children, m, m)
  File "/usr/local/lib/python2.7/dist-packages/pyangbind/plugin/pybind.py", line 742, in get_children
    parent_cfg=parent_cfg, choice=choice, register_paths=register_paths)
  File "/usr/local/lib/python2.7/dist-packages/pyangbind/plugin/pybind.py", line 1408, in get_element
    choice=choice, register_paths=register_paths)
  File "/usr/local/lib/python2.7/dist-packages/pyangbind/plugin/pybind.py", line 742, in get_children
    parent_cfg=parent_cfg, choice=choice, register_paths=register_paths)
  File "/usr/local/lib/python2.7/dist-packages/pyangbind/plugin/pybind.py", line 1460, in get_element
    element.search_one('type')))
  File "/usr/local/lib/python2.7/dist-packages/pyangbind/plugin/pybind.py", line 1150, in build_elemtype
    pattern_stmt = et.search_one('pattern') if not et.search_one('pattern') \
AttributeError: 'NoneType' object has no attribute 'search_one'

Could you check it, please?

TIA,
Lluís

[JoseIgnacioTamayo]

Update the Issue tiitle to reflect the intention.
Also, #284