Bugfix for potential app crash.

The crash occured when you:
- have used a workout before (so there were training entries)
- added a new exercise and
- wanted to continue your last training

As the new exercise had no history/training entries the app would crash later when trying to access the training entry.
Fix #34 (No icons for custom exercises)

Instead of a specific icon the first exercise image will be displayed as 'icon'.
This is not the perfect solution as the memory consumption may be too high.
Updated year in copyright hint.