Added initial unit test using CUnit.
Split tests into their own files for expandability.
Changed safeword_db_open to safeword_open.
Fixed segfault when either safeword_db, username, or password are NULL for safeword_credential_add().
Added initial unit tests for adding credentials.
Renamed safeword_strerror to follow convention of suffixing error functions with safeword.
Changed safeword_check to set errno_safeword; this will require refactoring for all safeword functions. Fixed broken init unit test.
Refactored perror and strerror for safeword errors to be prefixed by safeword_.
Refactored errno values; no longer fallback to errno.h..
Fixed issues reading credential information from db. Refactored safeword_credential_info to safeword_credential_read. Refactored message field to description.