Adds find method to StringMap

This could be useful if you need to locate an item in a string map but the value you want to check is a property of one of the items.

E.g.

``` javascript
var sm = basilisk.StringMap.from({
    a: {foo: 'bar'}
    b: {foo: 'nothing'}
});

var objectWanted = sm.find(function (item, key) {
    return (item.hasOwnProperty('foo') && item.foo === 'bar');
});
```