ImmutablePropTypes.contains

As referenced in #6, 'shape' causes problems with linters that are looking to validate props usage from React.PropTypes.shape descriptions. This release adds contains to replace shape, however it leaves shape in place for users who already have code working and do not care about linting with yannickcr/eslint-plugin-react.

v1.0.0

Tagging version 1.0.0 (stable).