Add script for computing translation probability table

[sylvainraybaud]

Features:

• fix build on mac and update README
• make it possible to read and write from and to gzipped files
• add mkmodel.py, a script based on align.py with the additional feature of computing translation probability from alignment
• add IBM1 module for representing translation tables, with basic tools (dump, look up, estimate)
• add script for printing simple stats about the translation table

Requirements:

• add dependency to convenience module