add Python 3.7 support

[superbock]

Changes proposed in this pull request

Enable Python 3.7 CI tests.

Furthermore, it fixes a couple of smaller issues:

• remove executable bit from some modules, thus doctests are not skipped any more
• fix doctests for the modules previously skipped
• enable testing of docstrings with py.test
• fixed some np.maximum warnings when working with NaNs
• use tuples for multidimensional indexing (to prevent future failures)
• remove deprecated testing asserts

Remaining TODOs before this pull request can be merged

However, enabling doctests for all modules lead to weird failures on Python 3.6 and 3.7. See https://travis-ci.org/CPJKU/madmom/jobs/442740662#L1167. However, this problem only persists for Python 3.6+ and is hard to nail down. Thus I disabled doctests for 3.6+.

• check if the failing doctests can be fixed; not sure what the actual problem is; cannot reproduce locally
• see if Cannot install with Python 3.7 #373 still persists and what the reason for this failure might be, cannot reproduce locally

[fdlm]

lgtm