initial commit for oandapy ebuild file
add support for python3 version of oandapy

The original oandapy repository by OANDA does not sport a version
compatible with Python 3. For that matter a fork was created that
contains the required changesets.
This change adjusts the ebuild to pick the original oandapy repository
for the Python 2 installation but switches to the fork for the Python 3
version.