cleanup OTAPI_Exec
Merge pull request #319 from mhamrle/otapi_cleanup

cleanup OTAPI_Exec
Restore OTLogStream::overflow, removed in commit 7deb336.
It broke the opentxs output to stdout/stderr (e.g. "opentxs list").
Also:
Revert "OTLog: -Wunused-private-field"
This reverts commit d09d187767eb251d1f8ff91bb1730dfe0a5fe7e2.