add os.walk, isstream, isstorage

[decalage2]

Idea from @bontchev:
would be nice to have the equivalent of os.walk in olefile. Also isstream(path) and isstorage(path) would be useful.

[decalage2]

Note: you can already check if a directory entry is a stream or a storage using get_type:
https://github.com/decalage2/olefile/wiki/API#get-information-about-a-streamstorage