Fixing Illegal Characters in Environment Variables

Tags are passed to event handler scripts as `SERF_TAG_{{NAME}}={{VALUE}}`. The `{{NAME}}` needs to follow the rules for valid environment variable names. Following the open group's spec on environment variables, this PR converts any chars not in [A-Z0-9_] into an underscore ('_') char.

fixes https://github.com/hashicorp/serf/issues/200

This is my first commit to Serf and I'm fairly new to Go. General feedback appreciated.