fix issue #635 (generated pdf contains only the address of an _io.BytesIO object)

[jeromecc]

Many thanks to Bogdan Petrea from Presslabs for his help, hints & lines of code.

[bogdanpetrea]

Thanks @jeromecc . The tests are passing but linting is failing apparently: https://travis-ci.org/silverapp/silver/jobs/489565179#L580
Also if you want to squash the commits, otherwise I can do it when I merge the branch.

[jeromecc]

Ok I see that you were not joking about code formatting. Linting is not joking about blank lines count either! :-)

Code style
The code should be properly formatted acording to PEP8, using 4 spaces.

I fixed my Eclipse setup to check for PEP8 shenanigans correctly. I will squash the commits. Hold tight.