SyntaxErrors in documentation

[gerritholl]

The examples in the documentation contain SyntaxError as they are still written for Python 2. This may erroneously make people believe posttroll is a Python 2 package, whereas in reality it is already Python 3 only (see #28). There are some other formatting problems in the documentation as well, for example:

from posttroll.subscriber import Subscribe

    with Subscribe("a_service", "counter",) as sub:
        for msg in sub.recv():
            print msg