Minor changes to be more robust and inline with the rest of the code

By un-exporting the TimePrecision type it becomes almost impossible to
wrongly use the type by people who would use this package as they now
can only use the exported constants Second, Millisecond and
Microsecond. The other change is only done to be more inline how all
other url’s are build up in code (they never use the fmt package for
that in this package, but just do string concatenation).