Add FDF support to mcpdf

[alokdhir]

Determine FDF vs XFDF by checking the first four bytes of input file.

[vog]

Thanks! This looks much better than #18. Remaining issues:

• Please keep parseArgs() simple. You don't need to change it at all. This function is only about parsing the user arguments, not about doing any actual work. For example, if there is an option that sets config.formInputStream to something different than System.in (e.g. a FileInputStream), that should be totally indepdenent of the fill_form option.
• Please put the "wrap around BufferedInputStream" logic into execute(), and put the FDF-recognition code into a new function isFDF(BufferedInputStream inputStream).
• Please separate purely cosmetic changes (such as re-indenting the "throws" declarations) from the functional changes. These should be two commits, or even better, a separate pull request, as this doesn't have anything to do with FDF/XFDF.
• In the README, please use uppercase FDF/XFDF instead of lowercase (except for file extensions, which remain always lowercase). Also, it may be good to state this upfront, so that the users sees right in the beginning of "fill_form" that this is about FDF as well as XFDF.

Note that it is sufficient to force-push your ammended commit to your branch. No need to open a new pull request. But do what's easiest for you. :-)

Thanks again for your effort! This is much appreciated.

[Jenselme]

@alokdhir This works great, thanks. Can you take into account @vog remarks so this can be merge?

[vog]

For your information: @MSerrano0410 took over and added stamping support (see #21), with the goal of polishing up FDF support after stamping. Unfortunately, it seems he got stuck when he was almost finished.

[vog]

Closing this for now. Feel free to open a new pull request if you want to contribute an improved patch!