fix summation and substraction from options

[schipiga]

This patch allows to increment and decrement variables within sections, inherited sections and conditional sections.
For example, you have a section, that creates a json like that:
{"key-1": "value-1", "key-2": "value-2", "key-3": "value-3"}

[make-json]
recipe = recipes:jsonize
json = key-1 value-1 key-2 value-2 key-3 value-3

and you want to add a part {"key-4": "value-4"} in Mac OS X:

[make-json:macosx]
json += key-4 value-4

It will not increment! It will create a separate invalid variable json + into .installed.cfg
And you will not be able to buildout again, because buildout will say: "Sorry, guy, json + is invalid variable format in .installed.cfg"