Implement free variable capture

[alex-sherman]

Motivated by the use of module global variables, like logging.Logger instances, deco should be able to detect free variable reference in its concurrent functions and pass in/proxy any it encounters. This functionality should be optional, perhaps even allowing the specification of exactly which free variables to capture.

A simple test is the following:

from deco import *

global_var = False

@concurrent
def my_conc():
    return global_var

if __name__ == '__main__':
    global_var = True
    result = my_conc().get()
    print(result) #Should print True, but prints False unless using concurrent.threaded