Added support to numeric mode

Since I needed support for both modes (I had a PDF417 barcode with some number codified numerically) I modified the original files to include it. Now the codewords for 900 and 902 are checked and depending on the mode the decode_number or decode_text functions are called.

I also had to modify the return of the content(vals) because my barcodes had an extra character at the beginning and I had to add a quiet zone "manually" since some barcodes didn't have any and the code always looked for it.