many changes: formats, modes, dtype, sf_open_fd(), ...

This commit holds several changes which were discussed in #22 (which was
itself based on #18).

Change handling of file formats:
File formats are handled as three simple strings: format, subtype and endian.

Add "which" argument to seek().
This is needed because the combination with logical or (e.g. SEEK_SET | READ)
isn't possible with string formats.

Update frames counter on write().

Hide all non-API names in module namespace (by prefixing _).
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

Get file extension from 'name' attribute of a file-like object.

Change public attributes of SoundFile class to properties.

Add 'name' property.

Add properties 'format_info' and 'subtype_info'.

Proper handling of dtype's in read() and write().

Add function default_subtype().