Add BrowserWindow.showDefinitionForSelection()

This API shows the system-provided pop-up dictionary.
Some Mac apps including Chrome have "Look Up in in Dictionary" context
menu item. This API can be used to implement it.
Merge pull request #938 from omo/show-definition-for-selection

Add BrowserWindow.showDefinitionForSelection()
Small style fixes
updated project name and icons.
updated product name and icon.
Merge branch 'master' of https://github.com/fireball-x/atom-shell