v1.8.1

Changes include:

• Use sanitize_title rather than sanitize_user to sanitize user_nicename; props @jrchamp.
• Changed activation and deactivation hooks to use __FILE__; props @jrchamp.
• Reverted to using $_SERVER in shibboleth_getenv() to handle use cases where getenv() doesn't return data; thanks to @jmdemuth for reporting.