mailer: Make delivery_method configurable

delivery_method is set to smtp by default. Users can
change it to sendmail like this:

    {
      "mailer": {
        "mail_from": "sensu@example.com",
        "mail_to": "monitor@example.com",
        "delivery_method": "sendmail"
      }
    }
metrics-net: Skip if /sys/class/net/:iface_path is a regular file

If the host has a bond interface, there is
/sys/class/net/bonding_masters, which is a regular file.
In this case, metrics-net.rb fails to run.
really basic ntp check.

this is pretty horrible, but i needed to replace a worse python plugin i was using. this is bad, but can be improved.
Merge pull request #373 from willejs/patch-2

really basic ntp check.
Merge pull request #372 from maoe/fix-bonding-error

metrics-net: Skip if /sys/class/net/:iface_path is a regular file
Merge pull request #365 from maoe/mailer-derivery-method

mailer: Make delivery_method configurable