Update to mock entry point detector.
Add concolic test case for invisible fields.

Crerated this test case while investigating the issues with the alaska-air example. There we fail to inject into a field which is in a "folded-away" part of the form which is not visible to the user. I was suspicious that we may not handle the case of non-visible (but also non-hidden) form inputs correctly.
Removed duplicated concolic test cases.
Add XPath entry-point selection to the demo mode.

For example, run
artemis --major-mode concolic http://www.rightmove.co.uk
and then click "set XPath for entry point" and try both "//input" and "id('buy')".

This is partly as a demo and partly in preparation/testing for adding the same feature to the concolic mode via the --concolic-button flag.
Merge remote-tracking branch 'origin/master'