Address some golint suggestions

- Error strings should not end with a dot.
- No need for `else` when if clause has return statement.
- Exported constants should be documented.
- Some minor changes about variables.

The following glint suggestions have not been implemented.
- Document all methods, or do not export them.
- Error variables should start with Err.

@eapache 
I'd be tempted to make these comments postfix rather than adding all the extra newlines