Add Player BindingCustomizer

[battila7]

Data that belongs to the current player (and not to its entities) is not exposed to the DSL at this moment.

A new DslBindingCustomizer should be introduced that provides an object that exposes getter methods that make it possible to access Player-level data. One example is the current resource level of the player.

[battila7]

This issue depends on #85 and #88.