Made go-odbc compatible with 'go get'.

* Moved sources to subdirectory that has a name, that can be imported
  (i.e. has no hyphen in it)
* Deleted build scripts (not needed, go get / go install will compile
  the package without additional build instructions)
* Also moved some documentation from README files to the packages, so
  they are accessible to godoc now.
Improved error reporting in driver.

When a (*ODBCError)(nil) error (returned by odbc functions) was returned
as an error, it was no longer treated as a nil value, therefore it was
not possible to detect them with the ususal `if err != nil {..}` method.