Guid prefix

There are two changes:

1. Introduce a constant for guid prefix. Good because it's less prone to errors, and neat if one wants to change the prefix (writing *em123* when debugging/developing is quicker than *ember123*, similar to the `Ember === Em` shortcut).

2. Reduces some repetition between `generateGuid` and `guidFor` (I'm unsure about this one). The current structure may be for performance, to skip an extra function call. Perhaps someone with more knowledge of JS performance can chip in?

(I'm happy to axe the second commit if it's more performant, and otherwise I'll squash them together)