Format and f-string support (and minor other changes)

[jorants]

Major additions

• Implements format() as per python standard
• Implements standard __format__ for all objects
• Implements __format__ for Str, Float, Int and Complex
• Fixes __repr__ for Complex by using the already present code from the __str__ method instead.
• __repr__ uses __format__ for float type
• Implements f-strings by manipulating the AST.

Small changes:

• Added generic_visit to ast.py. Unimplemented node types previously where ignored but now give an error.
• Added a fail in build-android.xml when android.jar is not found