* Implement consistent row addressing by Excel row number starting with 1

1. Added second versions for all row manipulation methods with zero-based row addressing.
2. Fixed methods documentation to explicitly describe which row addressing used in method.
3. Added WARNING to README.md.
4. Cosmetic change: All row test moved to file `rows_test.go`.

* TravisCI: go1.12 added to tests matrix

* BACKWARD INCOMPARTIBLE: Use only Excel numbering logic from 1 row

* README updated