cgen: Inititalize local variables correctly

Int variables get initialized to 0 implicitly, others to None.

Fixes #69.
icode: Globals and locals with explicit types

Closes #78.
icode: Fix test case
Fix #77: transform of implicit data attribute

The problem was actually in the type checker.
transform: Generate accessors for implicit attrs

Fixes #80.
Return 0 implicitly if function returns int

Fixes #81.
Merge pull request #79 from ashleyh/master

fix #52
Fix interface test case
Refactor: simplify VarDef (close #84)
cgen: Support basic inheritance
Add inheritance test cases
cgen: Refactor class info generation
cgen: Add inheritance test cases
Fix inherited __init__ in C back end

Closes #48.
icode: Calls to superclass __init__ (close #88)
icode, cgen: Calls to arbitrary super methods

Closes #91.
nodes: Update docstrings and comments
transform: Support basic operator overloading
transform: Support indexing (gets only)
transform: Indexed assignments