Change public attributes to properties
Fix a seek-related bug
Add close() method

Closes #14
Remove superfluous parentheses in return statement
Proper handling of dtype's
Hide all non-API names in module namespace

This is relevant for tools that do introspection, e.g. for
auto-completion in IDEs.
Support sf_open_fd(), remove obsolete argument virtual_io

The first constructor argument can now be:
- a string (path to a file)
- an int (a file descriptor)
- a file-like object

The argument virtual_io is not needed, because this can be detected
automatically.

Closes #19
Fall back to read() if readinto() is not available

See #19
Add 'name' property