Change 'r'/'w'/'rw' to 'r'/'w'/'r+'/'w+'/'x'/'x+'
Avoid catch-all exception handling
Use sf_open_virtual() instead of sf_open_fd() if file name is given
Shorter exception message on invalid file argument

Information about what is allowed in the "file" argument should be
written into the docstring, not into the error message.
Fix name property
Change seekable from property to method

... in order to be compatible with Python file objects:
https://docs.python.org/3.4/library/io.html#io.IOBase.seekable
Check if seekable() before calling seek()
Merge pull request #60 from bastibe/open-rw

Change open modes to be more in line with Python's open() function