Runtime Issues

I don't know if this is related or the same as #34, but instead of getting a crash, I am getting a runtime issue saying that 

>  Simultaneous accesses to 0x7f9c20c35e30, but modification requires exclusive access

on this line:

```
// PullToRefreshView.swift line 153
if !(context == &kvoContext && keyPath == contentOffsetKeyPath) {
```
