Better conformance with IPEP 27

Modifies return values so they return the JSON objects described in IPEP 27.  In particular, the 'name' and 'path' fields are made consistent with IPEP 27 when before 'path' was not always correct and both fields were sometimes missing.
Don't restrict to notebooks and directories, in list_contents

Before list_contents was (incorrectly) only listing .ipynb files and directories.  Now it lists all files and directories.
Merge pull request #49 from KesterTong/tospec

Better conformance with IPEP 27