Add ExponentialTimer helper class to util

Ignore-this: a6182a235b4f41a599d3056f06cb50bb

darcs-hash:20140908123129-56b4d-1c477f7a2cfc667d4380946daa4370c8b8c2cc9b
Use an ExponentialTimer to send a few keyframes after the stream starts

Ignore-this: 266f05d60e8260c6fd75e217700372e

darcs-hash:20140908123358-56b4d-9ca187ee7b8d51d372b5a8999cd2e81e262ecd78
Fix removing observer

Ignore-this: 853363c511e38d2b2a5dd6f570ae686e

darcs-hash:20140908152252-56b4d-59e1f17cb0f52034738143f3c99039717e6c0fab