fix release 0.2.4: include missing SUMMARY.txt in distribution

Apparently, 0.2.3 pip install was broken (see http://code.google.com/p/mdanalysis/issues/detail?id=168)
due to missing SUMMARY.txt.

The file is now included (via MANIFEST.in) together with README.rst.

The version number was increased to make automatic updates cleaner.

The only functional change in this release is the addition of the
top-level gridData.__version__ atttribute.