Include the overlapping xpath for ClickeFailed from obscured elements

This makes it easier to debug failed clicks because of modals, etc.

Example message:

    Failed to click element /html/body/div[@id='one'] because of overlapping
    element /html/body/div[@id='two'] at position 199, 199
Fix Node#path

* Also adds missing specs for capybara's Node#path
Fix failures from invisible nodes, path/pathForNode mixups