[lib] using Pod::Checker to check pod

Pod::Simple was missing many warnings and errors and thus has been replaced
by Pod::Checker.  This is sensible, since the advice given by the
`pod_syntax.t` test when errors occur is that one should use `podchecker` to
investigate errors in detail, thus we should be using Pod::Checker
internally to begin with.

My only issue with this patch is that I've added a dependency on `Capture::Tiny` in order to ignore the output from `Pod::Checker`; I hope this is OK.

Comments welcome.