fix bugs in update of offset/lock: missing function ref, bad scoping …

[hairylime]

…on watch and mount hooks

Tried to use the update of offset (on a screen resize event) and got a series of errors from the module:

• missing updateWatcher method on $scrollMonitor
• in vue, you can't use arrow functions on watchers if you need 'this' to resolve to the component
• beforeMount nextTick doesn't seem to work (_container never initialized), changed to mounted on ScrollContainer (since that is effectively 'mount started') and mounted/nextTick on ScrollItem ('mount finished')