Resolving properties

A decorator for Properties, which allows to usage of tokens in property string variables. 

For example:
```INI
[section]
val1=${prop.property.section.val} 
val2=${env.SERVER_PROFILE}
```

Also a default value can be supplied:
${prop.property.section.val|default}
If the token can't be resolved (missing property/missing env var) and there is no default value, an exeption will be thrown.
 


Why make this public? It will only ever get called by this class, so it can be protected.