General fixes + get it working on OSX

[MatsWillemsen]

When calling Index() failed, because Indri reports an error, do not PY_DECREF self->query_env_obj, because it is likely NULL (it gets created later), so use the safe PY_XDECREF.

Also, with clang (on OSX), it needs to be specified that the code is C++11, to understand some of the features in the code. On Ubuntu, it works out of the box.

[cvangysel]

Thanks for your contributions. To keep things organised, can you please:

1. Split up this issue in two separate issues (one for the installation fix, another for the reference counting fix).
2. Remove any unnecessary commits/changes from the pull requests (e.g., adding a Homebrew file and removing it directly afterwards is just noise, the comment you add at the top of setup.py has to go, etc.).