Enforcing a SECRET_KEY

The SECRET_KEY config will be used for signing the authentication token. Shiva
refuses to start if this setting is not defined.
Merge pull request #131 from tooxie/SECRET_KEY

Enforcing a SECRET_KEY
Abstracting away repeated logic
Dropped request.values usage

The dictionary `request.values` contains both querystring arguments and
POST/PUT body arguments. It's better to be explicit about where those values
should be coming from.
Merge pull request #132 from tooxie/dry

DRY
Smarter response

The `shiva.http.JSONResponse` object does some checks in case of error. The
function `flask.ext.restful.utils.unpack` was extended.
Merge pull request #134 from tooxie/unpack

Smarter `shiva.http.JSONResponse`
Improved shiva.utils.parse_bool
Merge pull request #133 from tooxie/parsebool

Improved `shiva.utils.parse_bool`
User model
User metadata
User resource
Documentation on Users
User tests
Token-based authentication
Added shiva-admin utility

This utility script will be used for some administration tasks like user
creation.