Added API function iio_buffer_start

This function can be used to get a pointer to the raw data contained
in a iio_buffer structure. It is useful, for instance, when one wants to
work on the raw stream.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Generate and install libiio.pc for use with pkg-config

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Added omitted Doxyfile

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>