Added support for break and continue. This resulted in a few changes.

First, "Render" is now called "Execute", which is more generic and more
representative of what might happen.

Second, Execute returns a core.ExecuteState which can either be Normal, Break
or Continue. These return types control the flow of execution. In almost all
cases, core.Normal is what you want to return.
Merge branch 'master' of github.com:karlseguin/liquid