Separate tmp files for every process/worker

[zauster]

Hey,

very helpful package!

I run simulations in Julia and use PDFmerger.jl to create multi-page PDFs with descriptive statistics of the simulation. I tried now to run several simulations on different processes/workers, which, however, resulted in errors in the appendPDF! function. It seems that the multiple processes are interfering with the temporary files of other processes.

So I created a quick fix in that the temporary files now have a identifier of the process that created them. I tested the change locally with my simulation with multiple processes and the fix seems to work fine.
The unit tests pass fine.

Cheers,

Oliver

PS: I also fixed some typos in the other commit.

[scheidan]

Great idea!
After I made a veritable mess with the merging I added your changes manually. I've mentioned your contribution in the Readme.