- added "--threads:on"
- fixed the Nim API tests and moved them to unittest
- added copyright header
- combined the 2 test file into tests/all.nim and set that as the
Nimble test target
- had Travis run that
- new "tempfile" dependency to create proper tempfiles + deleting them
at the end
- copied the error string to RocksDBResult
* error is a pointer to a cstring that the API will allocate on error
* if an error is returned, it must be free'd
* note missing tests for when error happens
