Allow specifying relative times in seconds

[csw]

Time values in seconds were not recognized as relative times in the same way as minutes, hours, etc. This produced surprising results; '30s' was passed directly to dateutil, which interpreted it as meaning 30 seconds into the current day, so awslogs displayed 15 hours of log data rather than the expected 30 seconds of it.

[coveralls]

Coverage remained the same at 92.069% when pulling ab3d34154d711ec5ecf20b309e3e94d317f4feed on csw:seconds into 401870b6cd055435d3dde5dd572e54165e1a4727 on jorgebastida:master.