Python 3 support

Currently using Python3 in wercker app causes an error with this step:

```
cd $WERCKER_SOURCE_DIR
(venv)$ export WERCKER_STEP_ROOT="/wercker/steps/wercker/email-notify/1.0.1"
(venv)$ export WERCKER_STEP_ID=""
(venv)$ export WERCKER_STEP_NAME="email-notify"
(venv)$ export WERCKER_REPORT_NUMBERS_FILE="$WERCKER_REPORT_DIR/$WERCKER_STEP_ID/numbers.ini"
(venv)$ export WERCKER_REPORT_MESSAGE_FILE="$WERCKER_REPORT_DIR/$WERCKER_STEP_ID/message.txt"
(venv)$ export WERCKER_REPORT_ARTIFACTS_DIR="$WERCKER_REPORT_DIR/$WERCKER_STEP_ID/artifacts"
(venv)$ mkdir -p $WERCKER_REPORT_ARTIFACTS_DIR
(venv)$ export WERCKER_STEP_TEMP="/tmp/$WERCKER_STEP_ID"
(venv)$ source '/wercker/wercker-build-essentials/init.sh'
(venv)$ mkdir -p $WERCKER_STEP_TEMP
(venv)$ export WERCKER_EMAIL_NOTIFY_FROM="#### "
(venv)$ export WERCKER_EMAIL_NOTIFY_TO="####"
(venv)$ export WERCKER_EMAIL_NOTIFY_USERNAME="####"
(venv)$ export WERCKER_EMAIL_NOTIFY_PASSWORD="$EMAIL_PASSWORD"
(venv)$ export WERCKER_EMAIL_NOTIFY_HOST="####"
(venv)$ export WERCKER_EMAIL_NOTIFY_ON="always"
(venv)$ source "$WERCKER_STEP_ROOT/run.sh"
File "/wercker/steps/wercker/email-notify/1.0.1/main.py", line 9
print 'There should be 8 arguments'
^
SyntaxError: invalid syntax
```