Moved ParseError to spdx so other parsers can use it without importing
each other.

Signed-off-by: Vlad Velici <vlad.velici@gmail.com>
Add metadata support, close #38

Signed-off-by: Vlad Velici <vlad.velici@gmail.com>
cli-tool: validate action for rdf format.

validator html enhancement: using <pre> tag for nicer indentation
in output.

Signed-off-by: Vlad Velici <vlad.velici@gmail.com>