Document execution language features

[gdaniel]

We need a wiki article explaining the capabilities of the language. This article should contain information on

• variable definition
• session access
• context access
• method invocation on variables
• arithmetic operators
• branches
• action invocation

[jcabot]

This is a first step: https://github.com/xatkit-bot-platform/xatkit/wiki/Xatkit-DSL-constructs

Needs to be improved but we can close this issue