fomosto: added travel time plots to tttview subcommand

optionally the source depth can be set using 
--source-depth=10
(in km)
otherwise the source depth will be the mid of possible source depths

![figure_1](https://cloud.githubusercontent.com/assets/1038455/4829317/3f9418ac-5f86-11e4-8fe7-c7260af86e1a.png)