move less files out of webroot to an "Assets" directory

- every plugin and the app itself get their own "Assets" directory
- LessMinFilter now looks for less files within the "Assets" directories
- the compiled output is still written to the "webroot/css" folder of the app or plugin respectively
add a JsConcatFilter for easy js library concatenation into one output file

- the "Assets" directory of the app or plugins can have a js directory where manifests are located
- manifests are simple *.js files that follow the rails asset pipeline syntax
- e.g.: "//= require subdir/subdir2/library.min.js"
- all require statements of a manifest equal one js file that should be included in the output file
- the output file is written to the "webroot/js" directory of the app or plugins and has the same name as the manifest file