Add Stream.close()

See comments in #14.
Less restrictive error handling in start()/stop()/abort()

start() is now allowed even if stream is already started.
stop()/abort() is now allowed even if stream is already stopped.
Improve callback signature
Update docs/examples regarding the new callback signature
Merge pull request #21 from mgeier/callback-signature

Improve callback signature
Get rid of _npsizeof

NumPy has a built-in dtype.itemsize attribute which can be used instead.
Merge pull request #28 from bastibe/_npsizeof

Get rid of _npsizeof
Clarify comment in close(), don't assign NULL
Merge pull request #27 from bastibe/close

Add close() and relax error handling in start/stop/abort
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