Before installing pysam, install cython

I think [this commit](http://code.google.com/p/pysam/source/detail?spec=svn8cbb117dd2928a04fadbf5066a28e178afa4434b&r=730d8b44f5efc2c9a2ad3ed75bea89f934659891) in pysam breaks installation on Python 2.6 and 2.7 without Cython installed.