Add Action type

[selenil]

Closes #24

[DynamicCake]

I just realized that the code in the docs for glua.try are wrong, it should be

/// use return <- glua.then(glua.call_function(fun, [glua.string("Hello")]))
/// use value <- glua.then(case return {
///   [first] -> glua.success(first)
///   _ -> glua.failure(Nil)
/// })

instead of

/// use return <- glua.then(glua.call_function(fun, [glua.string("Hello")]))
/// use value <- glua.then(case return {
///   [first] -> first
///   _ -> "Error getting first return value"
/// })