Add "setters" and "methods"

[jonathanvanschenck]

Right now, sets only write over the data at an endpoint -- it would be really nice to have the ability to do "setters" so that a set might actually call an internal function which could take action before returning the result. This behavior can currently only be achieved with a parameter-requiring function via a .get, which is a pretty strange way of accomplishing this task.

While we are on the topic, it is also strange that .get functions can take arguments to begin with -- in an OOP-analogy the .get functions are half acting like a member-variable and half acting like a member-function. I propose we split this behavior out:

1. .get's can be data, or a callback function taking no arguments (effectively a "getter")
2. .set's can either directly set data, or trigger a callback function (effectively a "setter")
3. We create a new operation .call or .compute or something which acts like the current .get function with parameters (effectively a class member function)