Shim script for extension handler

[ahmetb]

Wrote a bash shim that forwards logs of the handler process
reliably and daemonizes the 'enable' command via nohup .. &.

Adopted from https://github.com/Azure/azure-docker-extension/blob/master/scripts/run-in-background.sh and extended to be used for all commands (not just 'enable')

Fixes #35.

---

example pstree output

custom-script-extension install (attached to the process tree):

# pstree -ap
bash,1
  `-fake-waagent,79 /sbin/fake-waagent install
      `-custom-script-s,88 ./Extension/bin/custom-script-shim install
          |-custom-script-e,94 /var/lib/waagent/Extension/bin/custom-script-extension install
          `-custom-script-s,91 ./Extension/bin/custom-script-shim install
              `-tee,93 -ia /var/log/azure/custom-script/handler.log

custom-script-extension enable (detached from the process tree):

bash,1
  |-custom-script-e,157 /var/lib/waagent/Extension/bin/custom-script-extension enable
  |   `-sleep,158 100
  |-custom-script-s,138 ./Extension/bin/custom-script-shim enable
  |   `-tee,140 -ia /var/log/azure/custom-script/handler.log

cc: @boumenot

[ahmetb]

cc: @jasonzio as well, reviews appreciated

[boumenot]

LGTM.

[ahmetb]

merging but I appreciate if there are any comments afterwards.