[Tests] Removed Python 2 tests, Cleaned others

Removed the Python 2 specific tests. Cleaned and split some tests.
Modified ast_gen tests for PEP-8 and Pylint passing of generated code.
Modified test runner for better maintainability.
Updated _ast_gen to pass the tests

AST generator now passes the tests for it.
[LEGAL] Added disclaimer, Updated README.md
[API, TESTS] Changed syntax of AST nodes declaration

Changes:
 - Cleared '_ast_gen.py' for re-implementing.
 - Rewrote the AST declarations on the basis of Python's AST's ASDL
   definition.
 - Changed naming scheme for declaration files. They now use a ".ast"
   extension, preceded by the name of the language(s) they are for.
 - Changed API of '_ast_gen.py'.
 - Modified 'dual_ast.py' to assist the changes in following commit.
 - All tests for '_ast_gen.py' have been re-written accordingly.

TODO:
 - Write tests for 'dual_ast.py'
 - Implement '_ast_gen.py' completely.
[DESIGN] No more code generators..

As we only use 1 code generator, there is no need of an ABC.
No need to make an entire directory for one class...
[TESTS] Corrected certain mistakes in the tests

Certain ambiguities and errors in the tests have been corrected.
[TESTS] Switched to nose for running tests

All tests are now run by nose. Coverage is checked with the plugin for
nose. setup.py script now uses nose to run the tests too.
[WIP] Implemented ast_gen and dual_ast

Implemented code for ast_gen and dual_ast. They pass their respective
tests.

TODO: Need to add the tests for the dual_ast.py..
[LEGAL] Switched to GPLv3
[MISC] Added lextab.py to ignored and auto-deleted files.
Merge branch 'new_ast_gen_syntax'