Updated Tray to use libappindicator in Linux

I updated the code in **src/api/tray/tray.h** and **src/api/tray/gray_gtk.cc** to use [libappindicator](https://launchpad.net/libappindicator). I made this change because I could not get my Tray menus to display in Ubuntu. The changes only apply to Linux and have been tested in Ubuntu 14.04 x86_64.