raise an error if both size & buffer are passed

+ raise a ValueError if both size & buffer are passed

+ changed ValueError raised when size and buffer are None to a
  TypeError

Author: tgarc

src/pa_ringbuffer.py

@@ -85,9 +85,11 @@ def __init__(self, elementsize, size=None, buffer=None):
         self._ptr = self._ffi.new('PaUtilRingBuffer*')
         if buffer is None:
             if size is None:
-                raise ValueError(
+                raise TypeError(
                     "size is required when buffer parameter is not specified")
             self._data = self._ffi.new('unsigned char[]', size * elementsize)
+        elif size is not None:
+            raise TypeError('exactly one of {size, buffer} is required')            
         else:
             try:
                 data = self._ffi.from_buffer(buffer)