Change template.php to THEMENAME.theme

I'm surprised I couldn't find this issue in the queue, sorry if I'm duplicating.

For consistency and convenience, I propose changing template.php to THEMENAME.theme.

Reasoning

• Themes act a lot more like modules now (more so than .profile does) being able to implement hooks, etc.
• The .theme extension feels right as it matches sites/all/themes, just like .module and .profile
• Editing multiple template.php files in a text editor (say with cascading base themes) can get confusing.
• I assume there is no technical reason to stick with template.php