Using `+` to compose strings instead of `u''.format`

[ErwinJunge]

The code uses + to compose strings instead of using the modern python standard of u''.format
There are also some instances of % formatting.

We should convert all of the above to use u''.format.

https://docs.python.org/2/library/stdtypes.html#str.format

[iulyaav]

May I work on this?

[rohithasrk]

@Iuliaav There are already two pull requests corresponding to this issue. So, I think there is no point in duplicating efforts. You can work on some other issues in the repository. Check them out 😃

[iulyaav]

@rohithasrk Cool, thanks. I'll take a look at the other issues, see what needs a helping hand ^.^