Drop support for Intellij 11-era builds
Update icon and highlighter for Intellij 12+

Use DefaultLanguageHighlighterColors to get support for all default
color schemes, plus newer custom schemes which are correctly defined.
(See http://confluence.jetbrains.com/pages/viewpage.action?pageId=49463468
for more details).

Update icon design to play nice on both dark and light backgrounds.
Merge pull request #86 from dmarcotte/drop-11-support

Drop support for Intellij 11-era builds