add support for file_bytes argument with managed_file_context()

[Johnmaras]

Original PR camelot-dev#270

[Johnmaras]

That's my first across-repos PR so let me know if I did anything wrong.
Looks good to me though :)

[bosd]

I think you followed the right procedure.
Yet there are some merge conflicts here.
I tried to resolve them, locally by following the command line instructions mentioned here on a rebase.
After that I pushed them on a new branch.
But that created a lot of dirty commits..

So, Decided to give the cherry pick approach a go.
The merge conflicts and suggestions where a lot cleaner.