Use callback signature from cdef for finished_callback
Remove PaHostApiTypeId from cdef

The enum values are never used.
Also, for the use in Python, the host API names should be sufficient.
Remove superfluous indentation from code examples
A few formatting changes according to PEP 8
Set default blocksize to 0
Make frames obligatory in read()
Restructure the read() method
Merge pull request #30 from bastibe/misc

Miscellaneous Changes
added note about incompatible callback change
fixed critical bug in `read`