FIX: exit program on ctrl-C

[madisonmay]

Example to test with:

from tomorrow import threads
from time import sleep

N = 2

# a `timeout` value is required in order to ensure that 
# execution periodically returns to the main thread
@threads(N, timeout=100)
def hanging_thread():
    while True:
        sleep(1)

if __name__ == "__main__":
    results = []
    for i in range(N):
        results.append(hanging_thread())

    for result in results:
        print result

TODO:

• Add tests
• Decide whether this behavior is desirable