Switched the scrolling behavior to center the tooltip vertically

[jlovison]

Changed the scroll behavior to center the popover in the browser window, to minimize issues with popover placement and/or a higher z-index persistent navbar.

See issues #114 and #125

[jlovison]

@LostCrew I cut down on the variable use for half the window height, but not having much experience in coffeescript (and not having the compilation tools on my development machine), I didn't want to reduce down to a single line, as that animate call is technically a dictionary of CSS statements, and I could anticipate potential hiccups with the compiled JS.

[sorich87]

@kromem, install node.js (http://nodejs.org/). Run npm install -d command in the library directory to install the compilation tools and other dependencies and grunt build to compile.

[LostCrew]

@sorich87 @kromem integrated in v0.6.0