Document the special encoding value "bytes"

This took me some time to find out - better document it!
fix typo in documentation
die with helpful error if encoding cannot be determined
munge all found files in [PkgVersion], not a subset

A filefinder was already being used, but undocumented.  Also only .pm/.pl
files (or files with a shebang) were being munged, which excludes .pod files
-- only files with a package declaration will be modified, as before.

Closes PRs #219 and #233.

Conflicts:
	Changes
	lib/Dist/Zilla/Plugin/PkgVersion.pm