Limit to outgoing foreign key relationships in Database backends

Since graphlib 0.9.4, the bi-directional traversal requires specifying
the direction if both are not desirable.
Implement alpha backend for Solvebio API

The Solvebio API is changing quite a bit, so there is no guarantee
this backend will work as expected.
Rename REDCap metadata CSV file
Add sort order across backends for node types

This is implemented with a `sort` argument in the Node.definitions
method which calls `sort()` on the resulting Nodes tuple.
Support toggling hidden files and pattern matching in directory backend

The `pattern` argument can be passed which takes Unix shell matching. See
http://docs.python.org/2/library/fnmatch.html for details.

The `hidden` argument has been added which toggles if hidden files and
directories (prefixed with '.') should be included. If `hidden` is true,
hidden files will be shown.
Restore Node.__repr__ method for more informative output
Update iPython notebooks to reflect current state of the API
Fix REDCap default section label tests
Rename misspelled `Node.defines` to `Node.define` from 4dfb8272