Add support for PushBullet device_iden

ZNC Push currently does not support PushBullet device_iden, which is an alphanumeric field. It needs to be sent as parameter `device_iden` as opposed to the older, numeric `device_id`. 

Source: http://www.reddit.com/r/PushBullet/comments/1vof0d/ways_to_get_device_id/ceuaw3t

This pull request adds support for both, and changes the curl parameter based on whether `target` is alphanumeric or not. 

Example from testing:
```
[00:24] <*push> View the detailed documentation at https://github.com/jreese/znc-push/blob/master/README.md
[00:26] <rohan> set service pushbullet
[00:26] <*push> Note: Pushbullet requires setting both 'target' (to device id) and 'secret' (to api key) options
[00:26] <*push> Ok
[00:26] <rohan> set secret <API_KEY>
[00:26] <*push> Ok
[00:27] <rohan> set target udne6djAuXDo0g56
[00:27] <*push> Ok
[00:27] <rohan> send testing
[00:27] <*push> Ok
[00:28] <rohan> set target 5750085036015616
[00:28] <*push> Ok
[00:28] <rohan> send testing
[00:28] <*push> Ok
[00:28] <rohan> send testing2
[00:28] <*push> Ok
[00:29] <rohan> send testing6
[00:29] <*push> Ok
[00:29] <rohan> set target udne6djAuXDo0g56
[00:29] <*push> Ok
[00:29] <rohan> send testing6
[00:29] <*push> Ok
```

Before modification, the pushes with target `udne6djAuXDo0g56` don't work. After this patch, all the pushes work. 

Related issue: https://github.com/jreese/znc-push/issues/64