canonicalize libraries: script to convert from Apache format to Nginx

Merge of Vid's #533 with some fixes.
readme: reword the canonicalize_javascript_libraries note
paths: don't require existence

Create paths for FileCachePath and LogDir if they don't exist yet and give
them proper permissions.
Merge pull request #547 from pagespeed/jefftk-fix-dir-permissions

paths: don't require existence
merge commit