Aggregate changelogs from previous builds.

* added aggregated change logs to `TerasologyGameVersions`
* default is a lookback value of 1 for stable builds and 10 for nightly builds (hard-coded, not changeable yet)

Fixes #241
Cleaned-up `getAggregateChangeLog`.
Merge pull request #302 from skaldarnar/aggregate_log

Aggregate changelogs from previous builds.