Catch DateTime parsing errors

I had the problem that some of my PDF files had a Creation Date according to the american date format.
`pdf_info` should handle this case.

In the first step, it tries to parse it according to the american format. If this also fails, it just returns nil.

I also used the opportunity to 

1. strip the metadata key from whitespace.
  e.g. `     foo    :   bar   ` would return the key `foo` with the value `bar`
2. Wrote some more specs for `process_output`