Added instructions to README.md for how to setup and deploy the backend.

Added an admin-only handler to export the datastore to cloud storage.
Changed survey_type to just be a string -- EnumProperty and EnumField
were just too cumbersome when needing to convert between them and JSON.

Finished the taskqueue implementation of the data export. Had to write a
custom JSONEncoder to allow datetime encoding (ISO format). Currently
doing a less-than-ideal manual JSON writing routine, but it allows the
task to run with much lower memory overhead (it can write out a record
at a time to Cloud Storage instead of having to load all the records
into memory, and then process them into a dict and then into JSON).
Change .gitignore rule for backend/lib to be absolute from root of
repository.
Added more comments to export.py to explain some of the design
decisions.
Clean up lint errors.
Merge pull request #2 from christhompson/backend-export

Backend export functionality