Add @jasonlong's proposed icon

https://github.com/b4winckler/macvim/pull/44
Add Yosemite tab style
Copy PSMMetalTabStyle implementation & interface
Enable Yosemite tab style on OS X 10.10
Tweak tab bar appearance
More GUI tweaks
Fix offset on inactive tabs
Remove log statement
Remove whitespace
Change tab size on Yosemite
Remove +1 width on inactive tabs
Fix handling of window states when drawing tabs
Use Yosemite tab style for fullscreen on 10.10
Change truncating to truncate on head instead of tail

Makes more sense to truncate on head because of Cocoa name-spacing
prefixes
Fix syntax error
Use screen width instead of hardcoded values in tab bar control
Remove text shadow on tabs
Simplify isKeyWindow declaration
Add tab bar separator

Making it feel more like Xcode than Terminal (I don’t know how I feel
about this)
Correct color on tabs