Expression support in custom notifications

[fgalan]

Currently custom notifications support only a basic attribute value substitution mechanism, e.g:

The temperature is ${temperature}

It would be great to extend that mechanism to include aritmetic expressions, like the ones supported by IOTAgents, e.g,

The temperature is ${@temperature / 100 }

(IOTAgent expressions use ${@temperature} for plain substitution, which is different from the ${temperature} supported by CB. However, we could support both mechanism, to align with IOTA expression syntax and at the same time keep backward compatibility)