vm: Add basic implementation of END_FINALLY opcode.

Allows to have nested try blocks with except filters.

TODO: Don't add END_FINALLY's exception re-raise points to traceback.
Fix Makefiles: -O3 option was not being applied; mkdir for STM.
Merge pull request #240 from msiemens/windows_fix

windows: Make windows/main.c independent from unix/main.c
Merge pull request #239 from pfalcon/end_finally

vm: Add basic implementation of END_FINALLY opcode.
py: Add MICROPY_ENABLE_FLOAT around a float specific piece of code.