Delete requirements.txt

The actual requirements are specified in setup.py, these additional
requirements just add confusion.
See #82.
Remove SoundFile.frames attribute

There is already a more idiomatic way to get that information: len().

See #69.
Add MANIFEST.in

This adds the license file and everything from doc/ and tests/ to the
"sdist" tarball (which eventually ends up on PyPI).
See #80.
Merge pull request #82 from bastibe/issue-82

Change (or remove) requirements.txt
Merge pull request #69 from bastibe/issue-69

Remove either frames property or __len__() from SoundFile
Merge pull request #80 from bastibe/issue-80

Add MANIFEST.in
Add SoundFile.tell()

See #44.