Can't install to prefix other than /usr

[kopsis]

The code to load data files (en_US.json and main.glade) is hardwired to use /usr/share as the location. This prohibits alternate installation locations (e.g. /usr/local). The getDataDirs() function in tools.go can easily be leveraged to provide a "search path" for locating these files. I will submit a pull request that implements this change for your consideration.