Added `go get` and `godoc` compatibility and improved error handling in driver

* The code does now live in the subdirectory `odbc`, which does not have hyphens and therefore has an unambiguous import name.
* The driver now imports from `github.com/kch42/go-odbc/odbc` instead of `odbc` (if you want to pull the changeset, you need to change the `kch42` to `weigj`, of course).
* Since now every package import name is based on the github-URL, `go get` can automatically download and install the packages.
* I also moved some documentation from the README files to the package sources. `godoc` is therefore able to display this documentation.
* I also changed the error reporting of the driver, more details in the description of the second commit.