Add open(), read() and write() functions

See also #14.
read()/write() overhaul

Replace ffi.new() with np.empty() and np.ascontiguousarray()

Remove dicts for readers/writers

read():
* reserve only as much memory as needed (if 'frames' is too large)
* check return value of sf_readf_*()

write():
* avoid copy if data has already the correct memory layout
* check return value of sf_writef_*()
* don't return the number of written frames!
  This is for symmetry with read() and it's redundant information anyway

As the written frames are now checked in the write() method, the same
check was removed from the write() *function*.
Add arguments channels_first, always_2d, out and fill_value

See also #16
Add blocks() method and function