TagsComponent and TagsProcessor addedAdding a TagsProcessor and TagsComponent for processing tags. The Tags component was already available in the model but not in engine
Some minor changes in VarsContext so complex types (e.g. Entity) can be used for declaring variables
VariablesSystem updated so a "$this" entity variable can be used to set context.
Other systems and ChangeVarExecutor had to be updated since they use VariablesSystem
HasTag operator added to expressions. Test updated
@anserran comments addressed
Merge pull request #376 from jtorrente/expression-improvements

Improvements to expressions and variables system