Implementation of nodes as values.
Made getattr parsing uniform. Previously, it was patchwork for various
types of expressions. Cleaned up the treetop file a bit.  Modified
script_call mechanism (@ calls) to always return a hash.  Previously,
if it was a single entry, just the hash value was returned.
* Added indexing e.g. [1,2,3][2]
* Moved constants to const.rb
* Added Hash comprehension
* merged node_name and class_name in treetop. Should have been the
  same.
* Added engine.evaluate_attrs_hash function
* Refactored symtem defined functions
Removed INDEX function as its functionality is built into Delorean.
Changed enumerate_attrs_by_node to return a list instead of a hash.
The algorithm is now more efficient. Also, it no longer works if node
is nil.  Added NUMBER and STRING functions.
bumped up activerecord version to 3.2.12.