Blob: implement the memory buffer interface

[carlosmn]

This allows us to expose access to the blob's data without the need to
copy it into new buffer.

Should we take Blob.data and make it return a memoryview of the data instead of creating two ways o accessing it? I'm not too familiar with any differences in how you might want to use a PyBytes vs a memoryview.

[jdavid]

+1 to promote the efficient way. There is still Blob.read_raw() to get the bytes.

Only concern is memoryview is not available in Python 2.6

Let's do this for now:

• Keep Blob.data the way it is for now, so we don't break anyone's code this round.
  Just the docstring should be updated to warn about the future change.
• The tests won't pass with Python 2.6, so drop official support for it (and 3.1 by the way).

I am going to merge this and change the README file accordingly.

[jdavid]

Okey, the new support policy for Python 2.6 could read something like:

We won't actively break Python 2.6, and will accept fixes for it. But we
won't actively maintain it either.

(It looks like our support for Windows I would say...)

[carlosmn]

Sounds good. We might be able to expose a method that calls PyBuffer_FromObject() for python 2.6 in case someone absolutely can't move even to 2.7.