need better handling client/server time skew

[femto113]

Because the API signing algorithm relies on a time stamp it's possible to get errors like these when client and server times diverge (I've seen this when they're off by ~2 minutes, which can easily occur on EC2 instances).

{
  "system_error": "2015-01-22T19:20:14 api_ts parameter too far in the past or future",
  "request_id":"XXXXxxxxXxYXXX784XX",
  "server":"rmsprod2.la.sol",
  "datetime":"2015-01-22T19:20:14"
}

Since the purpose of api_ts is to serve as a anti-replay-nonce rather than an absolute time location, when this error occurs the module should probably compare local and server time and apply an adjustment factor to the timestamp to align it with server time before signing further requests.

[femto113]

Note also that those errors come back as Content-Type: text/html so may not be getting passed out through callbacks in the same manner as other errors.