Use subprocess rather than commands.

Also allow programs to be overriden with environment variables
($PKG_CONFIG and $PYREXC).
Build with Cython rather than Pyrex.

Cython is effectively an extended version of Pyrex; it seems to be more
widely used by other modules these days.
Update the translated libstatgrab API to match 0.90.

The bindings code still needs updating to make it actually work.
Have Cython generate the "statgrab" module directly.

This avoids wrapping everything twice (once in _statgrab.pyx and then
again in statgrab.py).

It should also make Python 3 support more straightforward, since
Cython's C output works for both 2 and 3.
Export the constants without the py_SG_ prefix.

"statgrab.ERROR_PARSE" is a reasonable kind of name.
Update the wrappers.

This cleans up the API to remove the SG_ and sg_ prefixes everything
used to have -- e.g. you now do statgrab.get_page_stats().
Move the libstatgrab low-level bindings to a separate file.

This means that the sg_ namespace is available in the statgrab module
for backwards compatibility.
Add wrappers to provide the old pystatgrab API.