Return extra error for not implemented features

In order to detect br0ken vs. unsupported archives,
return an extra error type for this to allow test driven development
and avoid confusing the user.
Remove owner/group support

Since owner/group has no meaning for modern use cases of ar archives,
remove its support in order to parse things faster and keep API small.
implement public interface

Basic archive reader
refactor archive magic check

Let it return an error, so we can pass it on.
Make invalid archive header an error.
Use the now more useful checkMagic

Remove duplicated functionality, since it now does what we want.
incorporate feedback from golint
Rename error types to have Error as suffix
unexport magic values
better error message

Since the per file magic contains an newline character,
it should not be included in the error message.