future imports fix

[dosmoc]

Accumulate and add compiler flags to calls to the compile function for eval/exec code in handleEval when features that alter syntax are imported from the future module. The problem was first reported in issue LightTable/Python/#23. More details are in LightTable/Python/#33. Allows the following code to be executed using Python2 without a syntax error:

from __future__ import print_function
import sys

print('spam', file=sys.stderr)

Does not resolve all of the problems with printing to sys.stderr mentioned in LightTable/Python/#23, just the SyntaxError.

[kenny-evitt]

@hayd @UnknownProgrammer Mind reviewing this?