Feature/grunt bower

We decided to use this weekend to do a little sprint to push mailvelope development a little further. We wanted to integrate a decent build workflow using grunt and add dependency management for ui libraries using bower.

While we were at it, we not only ported most Makefile targets, but also introduce new ones, like jshint. We also changed the directory structure a little. No code is generated into any subdirectory of source files any longer, but we build into the `build/` directory instead. This caused some confusion so we decided it would be better to clean that up.

We did no behavioural changes, so everything should work as before. The only reason to touch some source files were to fix jshint errors. (Basically white-space issues and missing semi-colons.)

TODO (still some issues):

* [x] switch back to original versions (we wanted to update, but we will do that in a separate PR)
    * I force-pushed, so the versions appear to be the correct ones, from the beginning
* [x] switch to bootstrap from bower
* [x] switch to modernizr from bower (or may be use grunt-modernizr to create a custom build of the lib)
* [x] ~~switch to wysihtml5 from bower~~
    * this has been shifted to another PR, since it seems to much work, for now
* [x] switch to spectrum from bower
* [x] switch to kendo-ui from bower
* [x] fix closing frame (seems broken)
* [x] fix `Blocked script execution in 'about:blank' because the document's frame is sandboxed and the 'allow-scripts' permission is not set.` when closing error message - check `common/dep/jquery.ext.js`
* [x] update development howto in readme file (npm install, grunt tasks, …)