FIX: eval in-page-script.js instead of injecting it using a script tag

This small change should fix #225 

NOTE: Unfortunately *evalInWindow* will be dropped by Firefox >= 34 (https://bugzilla.mozilla.org/show_bug.cgi?id=1042840), in the mean time I'm working on a better solution then using unsafeWindow.eval (which is too risky to be called on every navigated webpage by the user browser).