Add tests for linkages between daru & daru-io calls #55
Description
These tests are required, just to ensure that daru calls are redirected to appropriate daru-io calls. For example, I manually found out one such linkage bug while trying out Importer calls from Daru::DataFrame. Seems like I had missed to make the changes in PR #52 and have subsequently add them with fd08213 before release of v0.1.0 (fortunately).