Added retry, rndis, and tracing.

Merge of several features:

1) Add retry logic:     https://codereview.appspot.com/94650043
     Includes an overall agent refactor re: scheduling.
     Enabled by default, with per-run retryLimit of 1

2) Add network check:   https://codereview.appspot.com/90340044
     Verifies that we can ping the LAN gateway.
     Enable via command line:  --checknet yes

3) Add usb tethering:   https://codereview.appspot.com/90340044
     Uses murmur hash to generate the MAC address.
     Enable via command line:  --useRndis yes

4) Add chrome tracing:  https://codereview.appspot.com/102500043
     Enable via task:  trace=1

5) Avoid memory errors: https://codereview.appspot.com/80640044
     Refactor to reduce OutOfMemory errors