Catch DateTime parsing errors

[tmaier]

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

[tomtaylor]

Thanks! I rebased and merged this.