Fix to not block during metadata parsing

[fcamblor]

If metadata parsing error occurs, don't block indexation and consider the metadata as empty

[kimchy]

Maybe we can have a configuration flag to have the parsing lenient, and only ignore it in that case?

[fcamblor]

Why not, unless we have a workaround for #15

Wondering what should be the default value for this flag :

• lenient deactivated in respect of backward compatibility
• lenient activated to avoid having something which works while file's date is well formatted, and fails relatively silently when the date is badly formatted.