Handle invalid actions

• Handle invalid actions by returning the current state and a reward for picking an invalid action
• Valid actions also get an extra reward

v0.3.1: Merge pull request #11 from genyrosk/update-imports

Make environments available for import directly from root module e.g. from gym_chess import ChessEnvV1

ChessEnvV2: the power of Rust

Release of the v2 version of the environment with the core engine implemented in Rust

• over 100x performance improvement over v1
• major bug fixes in both v1 and v2 environments
• ported tests to the new environment

ChessEnvV1: big improvements

The new ChessEnvV1 is a major upgrade to the original environment (renamed to ChessEnvV0) and comes with an almost complete rewrite of the original code, with major improvements to code maintainability and readability, as well as bug fixes.

Full details: #8