Pure Python (2.*) fallback

[bwesterb]

There are several cases where the Cython module cannot be used.

• When using PyPy.
• On Google App Engine or another restricted environment.

For those cases it is useful to have a pure python implementation of msgpack.

I could release this as a separate project, but I prefer it merged upstream: it will be easier to keep the API the same.

There is another project which implements msgpack in pure Python, ( keisukefukuda/msgpack-python-pure ) but that is not a drop-in replacement.

Notes

• Setup will automatically skip the module if it fails to build. Maybe this is not desired: it could be moved behind a commandline flag.
• msgpack will first try to import the native module. If none is available, it will use the fallback.
• The fallback module passes all current tests (on Python 2.*)
• The fallback module does not yet work on Python 3. (That is why Travis complains.) I will look into that.
• The fallback module has been written with compatibility as first priority. There is a lot opportunity to improve performance.

A benchmark

As a very rough benchmark, I timed the pack and unpack time of a 30MB msgpack file from one of my applications.

1. Cython module 0.34; 0.24
2. Python 15.4; 10.77
3. PyPy 1.9 1.37; 0.43
4. PyPy dev-version 1.26; 0.40

[methane]

Very thanks to this PR!
I'll check, merge and release this in this week.

[bwesterb]

Thanks for the improvements!