Add mouseover and mouseout commands

Added LinkHints.activateModeToHover and LinkHints.unhoverLast to
perform hovering. These are unbound by default.

Also call LinkHints.unhoverLast automatically before hovering/
clicking the next link, to avoid permanently showing hover effects
(eg. IMDb watchlist grid view mode).
Move a private variable inside a do function closure
Skip over tiny frames when calling focusThisFrame

GMail and Google Inbox both use a large number of small (typically 0x0
or 1x1 frames) which we have to pass over with nextFrame to get to the
frames we want. We avoid this issue by ignoring all frames with height
or width < 3, since these will be essentially invisible to the user
anyway.
Use a tolerance when checking scrolling to fix zoomed scrolls
Merge pull request #1324 from mrmr1993/add-tolerance-to-scroll-check

Fix smooth scrolling when zoomed.
Update to 1.47
Fix smooth scrolling, all cases.
Merge pull request #1331 from smblott-github/fix-smooth-scrolling-properly

Fix scrolling in 1.47, all cases.
Update version to 1.48
Merge branch 'skip-tiny-frames' of https://github.com/mrmr1993/vimium into mrmr1993-skip-tiny-frames
Add comments to nextFrame logic.
Do not use Math.sign().
Merge pull request #1336 from smblott-github/do-not-use-math.sign

Do not use Math.sign().
Add a little more context to a comment
Revert "Hide the smooth scroll option from the settings UI"

This reverts commit 5ec7f490b5e9e68bddb415b66b6cae86841a3d52.
Increment version to v1.49
merge from philc/master
Merge remote-tracking branch 'mrmr1993/mouseover'