Add app_env var for script timeout

js_driver:eval_js/2 uses a macro SCRIPT_TIMEOUT for the amount of
time to wait for a js script to execute before timing out. In some
instances (testing JS map reduce on limited hardware) it would be
handy to set this timeout higher.

Use the 'erlang_js' app_env var 'script_timeout' and any int value.
Timeout is in milliseconds.