Fix install

Just tried to build and test dumbo on a clean Debian Wheezy, but some dependencies were not listed in the setup.py. Furthermore, those Python packages require system packages in order to compile, so I took the liberty of listing them on the README.

I hope this complies to your style. Any suggestion is welcome.

I tested the changes by cleaning the buildout, and re-issuing:

```console
$ python bootstrap.py && bin/buildout && bin/test
# ... buildout output skipped ...
..............reporter:counter:Program,test_counter,1
reporter:counter:Program,test_counter,1
.reporter:counter:Program,test_counter,1
reporter:counter:Program,test_counter,1
.reporter:counter:Program,test_counter,1
reporter:counter:Program,test_counter,1
reporter:counter:Program,test_counter,1
..reporter:counter:Program,test_counter,1
reporter:counter:Program,test_counter,1
reporter:counter:Program,test_counter,1
reporter:counter:Program,test_counter,1
.............
----------------------------------------------------------------------
Ran 31 tests in 0.628s

OK
```