Fix IE10

As reported in #3 I'll try to fix idb.filesystem.js for IE10.

This first commit only removes the uses of \_\_defineSetter\_\_ and \_\_defineGetter\_\_, I'm still getting a ```INVALID_MODIFICATION_ERR``` which I've yet to debug, but I wanted input on the following:

Is there a reason you need the getter/setter anyways? Why not use the prototype? [This](https://github.com/ebidel/idb.filesystem.js/blob/master/src/idb.filesystem.js#L169) is the only use of a setter where it's (under my impression) a sensible thing to do; are there problems if the properties are not defined with setters?