Provide API for building AuthOptions from env vars.

Addresses #112 .

AuthOptions fills out a gophercloud.AuthOptions structure with the settings found on the various OpenStack OS_* environment variables.  The following variables provide sources of truth: OS_AUTH_URL, OS_USERNAME, OS_PASSWORD, OS_TENANT_ID, and OS_TENANT_NAME.  Of these, OS_USERNAME, OS_PASSWORD, and OS_AUTH_URL must have settings, or an error will result.  OS_TENANT_ID and OS_TENANT_NAME are optional.