add support for numpy 1.8+ data types for conversion to r dataframe

Numpy changed the names of their data types in 1.8
http://docs.scipy.org/doc/numpy-dev/user/basics.types.html

specifically, bool, int, float, and complex are now bool_, int_, float_ and complex_

this requires changing the VECTOR_TYPES and NA_TYPES dictionaries used by convert_to_r_dataframe in pandas/rpy/common.py

numpy 1.7 isn't feasible to include as a project dependency (too many packages require 1.8)

Unfortunately, bool_ etc don't exist in numpy < 1.8, so you'll have to try/except that bit. I can change it and submit a pull request if you like
use LooseVersion here instead