Allow timedelta specifications in add_timeout

[rbu]

The timedelta API makes this easier to use (and test).

[bdarnell]

Good idea. I had thought about doing something like this if the number passed in was in the distant past, but it always felt ugly to have some magic number at which everything changed. Using timedelta objects makes it unambiguous. I've committed a slightly different version of this to use the total_seconds() method instead of converting it to seconds by hand. I also considered adding support for datetime objects for completeness, but converting datetime to timestamps is a pain so I'm leaving it out for now.

[rbu]

total_seconds was introduced in Python 2.7, is this version limitation intented?
http://docs.python.org/library/datetime.html#datetime.timedelta.total_seconds

[bdarnell]

No, it's not intentional. I'll change it to compute the total manually.

[rbu]

Just for reference, this was fixed in 6b7ddc9