fix minitwit/flaskr test errors, improve docs about file open mode

app.open_resource needs to get called with the correct mode param (python3
will read bytes [not str] if the wrong mode is used), add mode param docs.

rv.data is bytes, fix the data type we compare it with to be also bytes
Merge pull request #742 from ThomasWaldmann/sprint-branch

fix minitwit/flaskr test errors, improve docs about file open mode