Fixes  #660 - Thanks Julien Moumne for the patch! great new feature improving Piwik responsiveness and load time.
 * Added new writable dir /tmp/assets/ to write the merged CSS/JS
 * I had to update checkDirectoriesWritableOrDie and added a slash at the end of all paths, or directory creation in checkDirectoriesWritable was failing

git-svn-id: http://dev.piwik.org/svn/trunk@2392 59fd770c-687e-43c8-a1e3-f5a4ff64c105