Ensure long description is Unicode string in setup

Python 3.3 is not able to properly install Falcon
due to UTF-8 characters in the README.rst file. The
characters are fine, we just need to be sure to decode
the open('README.rst', 'rb').read() bytestring as a
unicode string.

An example of the setup failure can be seen here:

UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 586: ordinal not in range(128)

http://logs.openstack.org/40/55040/2/check/gate-solum-python33/02f9aca/console.html