Extracting raw images hit an error because exposure
time is now returned as a Rational.  I assume an
underlying library changed or something!
I don't have time to investigate, but I now handle
TypeErrors by attempting to use the value
directly - this does seem to give the right behaviour.

I have also included command line flags that allow you
to disable the EXIF extraction in case it fails in the
future.

okh: Fixes a white-space issue

See merge request !2

There was one TAB character, which is not allowed in strict YAML,
even though some parsers accept it.

The updated picamera fork returns a memoryview object for the
lens_shading_table property, in order to avoid being numpy-
specific (a request from the maintainer of picamera).  This
commit explicitly converts the table back to a numpy array
before dumping it to the YAML settings file, which keeps
behaviour consistent between the two versions.

This fixes #8