Use `assets/images` instead of `static/img`.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	assets/images/bg.jpg
	bbs/content/style.css
	bbs/editpid/index.php
	bbs/edituser/index.php
	bbs/home/index.php
	bbs/index/style.css
	bbs/lib/bg.jpg
	bbs/login/index.php
	bbs/main/style.css
	bbs/register/index.php
	bbs/search/index.php
	bbs/user/index.php
	static/img/bg.jpg
Use `assets/images/static` instead of `assets/images`.
.gitignore: exclude `assets/images` except `static` subdir.
Merge remote-tracking branch 'upstream/master'
Fix the background problem in long thread.
Merge remote-tracking branch 'upstream/master'
Fix all repeat problem in background.

But actually we should redesign the whole structure.
Why do we need so many places to set the same background?
Merge remote-tracking branch 'upstream/master'
Merge pull request #32 from huxuan/master

New better background images.