Added app specific sound support and cleaned up formatting of the Pushover message

Made these edits to improve my pushover-weechat experience.

Sounds can now be selected using `/set plugins.var.ruby.pushover-weechat.sound`

signal_data from weechat is split into username and message before being sent to Pushover. Pushover message title now includes sender username while pushover message contents do not (see attached image).

![weechatchanges](https://f.cloud.github.com/assets/31131/1285846/aa0a1088-2fb7-11e3-95ce-11b950702e92.png)