New report field: 'STACK_TRACE_HASH'.

Adds a 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