Merge pull request #21 from aaron0browne/master

Remove redcap section header props
Remove io/exporter capabilities from Origins

This is out of the scope of Origins. In addition it's immediate capabilities
are supported by graphlib.

Updated graphlib to 0.9.4 which supports bi-directional traversal of
relationships.
Add back `element_preceding_header` in SELECT for REDCap MySQL backend

This was removed in b435bb82 to prevent redundancy in the field attributes,
however the value is still needed for filtering. The attribute is now removed
from the attributes before returning.
Rename CONTAINS to DEFINES

Fix #22
Rename _contains and _containers to defines and definitions
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
Add initial support for the Harvest API backend
Rename generic RELATES relationship to REFERENCES 

Although REFERENCES is still generic, in this context it is more
descriptive since these are pertaining to foreign key references.
Ensure the postgresql user and password are None if empty
Pass the `sniff` rather than the hard-coded value