export ANSIBLE_HOME so it can be used in scripts

In order that scripts like this can work 
```
#!/bin/bash
ansible -vvvv tag_instance_type_foo-training -i "${ANSIBLE_HOME}/plugins/inventory/ec2.py" --private-key=~/Downloads/foo-training.pem -u ec2-user -m ping
```

hacking/env-setup

@@ -13,7 +13,7 @@ fi
 # The below is an alternative to readlink -fn which doesn't exist on OS X
 # Source: http://stackoverflow.com/a/1678636
 FULL_PATH=`python -c "import os; print(os.path.realpath('$HACKING_DIR'))"`
-ANSIBLE_HOME=`dirname "$FULL_PATH"`
+export ANSIBLE_HOME=`dirname "$FULL_PATH"`
 
 PREFIX_PYTHONPATH="$ANSIBLE_HOME/lib"
 PREFIX_PATH="$ANSIBLE_HOME/bin"