Check maturity for valid values

[cornelius]

The maturity values should be from a set of predefined values, so that we can automatically detect and compare the status of different releases and libraries.

The following values should be legal:

• prealpha (development version before the first release)
• alpha (release of unstable code)
• beta (initial releases of stable code for gathering feedback)
• stable (production-quality code)
• edge (development version of released library, the bleeding edge)