New report field: 'STACK_TRACE_HASH'.
This field contains a hash of the reported stacktrace, facilitating grouping of similar crashes.
As discussed here: https://groups.google.com/forum/#!searchin/acra-discuss/bod/acra-discuss/DajoZbpO-4w/Fvd1LVx_09sJ
Merged PR#89 branch 'feature_stackTraceHash' of https://github.com/BoD/acra into merging_89
Removed unused import.