add absolutePaths option to CFileHelper::findFiles

if absolutePaths is true (by default), function will return absolute paths, otherwise the relative ones
mbstring fallback for CFileHelper::findFiles
code conv fix, charset specifying and docs improvement
Merge remote-tracking branch 'upstream/master' into patch-1
CHANGELOG updated
Merge remote-tracking branch 'upstream/master'
another way to return relative paths: findFilesRecursive main loop instead of findFiles post-processing
Merge branch 'master' of git://github.com/yiisoft/yii
Merge branch 'master' into patch-1

Conflicts:
	CHANGELOG
CFileHelperTest.testFindFiles_absolutePaths was added
CFileHelperTest.testFindFiles_relativePaths was added
Merge pull request #2722 from pavel-voronin/patch-1

add absolutePaths option to CFileHelper::findFiles