Add function for defining constant properties.

Ignore-this: 2f49aa6b9bcac6acc23dec64c8c006c5

darcs-hash:20140603204028-4d2ae-f33ede06e1a7d439ac65de5eeb1781ef9a6f4fc2
Increase version to 0.3.1.0.

Ignore-this: 7812b78a58bf26d8ba2a962654030fa3

darcs-hash:20140604234722-4d2ae-8d117ae62b74864a4073d04f81433b26322c02db
Fix intermittent crash on exit when pending signal outlives engine.

Ignore-this: 99c733d8594f208ff3c0ec766eb4c6d

darcs-hash:20140604235128-4d2ae-36de47580e82050d8fd9bad9b5f817b0a3601b4e