lock down whats considered a valid function string

- string arguments are first trimmed
- empty strings are cast to null
- strings must begin with "function" or casting fails
- all string content after the function closes is ignored
- only valid function strings, Code instances, or functions are allowed
- function args are no longer accepted (used to be passed to new Function)

We are validating function validity but not function content. Side
affects of function execution are not guaranteed to be safe.

**You are responsible for validating the function is safe
for execution.**

If function content is updated in the database to something that
does something nastly like `rm -rf /`, and you run the returned
function, you've been warned.
allow user defined fn conversions

var module = require('mongoose-function');
module(mongoose, { toFunction: yourConverter })
update README
expose toFunction
release 0.1.0
Merge branch 'master' of github.com:aheckmann/mongoose-function