Reset scrollbarwidth cache when window is resized

Zooming can produce different measurements.
forgive one pixel in scrollbars

Sometimes I find that CodeMirror adds scrollbars inappropriately (most often due to browser zoom).
Digging around suggested that it is some non-integers resulting in rounding up the measured size,
and I haven't been able to find a case where the two values differ by more than one pixel, so simply setting the threshold one pixel higher seems to address the issue.

This PR forgives one pixel in the size comparison before drawing the scrollbars.
Add handleMouseEvents option to markText and addLineWidget
Take top padding into account in "local" coordinate space, fix bug in fromCoordSystem

Issue #1557
Add a lineAtHeight method

Issue #1556
[vim keymap] Fix reliance on bug in coordsChar
[closebrackets addon] Add 'explode' feature

Issue #1552
[show-hint addon] More complete documentation
[show-hint addon] Make sure to clear state.completionActive when no match is found
Add support for "window" coordinate system to charCoords/cursorCoords
Remove explicit compensation for paddingTop in scrolling code

Local coords now include paddingTop.

Issue #1557
Another test that fails mysteriously on Travis' Phantom

(but not on my local one, or in a real browser)
Merge branch 'master' of https://github.com/marijnh/CodeMirror