get_site and get_request limitations

[zupo]

Quoting davisagli: "Regarding the calls like get_site_url() and get_request(), it's a bit misleading to say that these will always be available. It would be good to warn that the API is intended for use within code published by the ZPublisher (which sets up the necessary thread locals), as opposed to external methods or code run via bin/instance run or whatever."

We need to make a list use-cases when get_site and get_request will not work so we can put it in the documentation of the methods.

[miohtama]

We need to make a list use-cases when get_site and get_request will not work so we can put it in the documentation of the methods.

Also raise rather verbose AssertationError("You cannot do this, because X").

They are more helpful than ComponentError: Unknown look-up exceptions.

Plone methods have bad habit never sanitize input.

[zupo]

Yes yes yes! And more yes yes yes!

[zupo]

Discussion continues in #20.