adding support for . path resolution in context get


The changes are primarily in what it is searched in the scope

1) If path not found in the current context head, search next level up in context (ie in tail.head)
2) If not found in top context (tail.head===null), then search in Context's this.global

This change relates to how the path is referenced

3) If a value in a path is a function, apply the function before getting path element.  eg, foo.funcbar.baz - would call apply on function 'funcbar' before getting property baz.  Note that in the expression foo.funcbar that the function is not evaluated - functions are only evaluated if found before the last element of path. 

In my opinion, this make data referencing consistent.  

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Also see issue #270

This code has a few new tests and passes all existing tests.  

If you want to use this, you can simply include this gist after your dust import: https://gist.github.com/carchrae/5825054