Use /acad-year/sem/ for route prefix

This is the start of a series of patches to support using different
academic year and semester for the timetable builder.

We use a route format similar to NUSMods, like this:

    YYYY-YYYY/semX/

where `YYYY-YYYY` is the academic year, such as 2013-2014, and `semX` is
the semester, such as sem2.

Other changes required:
- For the `index.html` page, we prefix all the css and javascript paths
  with a '/'. This tells compojure to look at the `resources/public`
  folder for those files, instead of looking at the YYYY-YYYY/semX
  folder.
- Editing the urlPrefix for Exhibit3. For details, see the diff for this
  commit. The file is `src/js/vendor/exhibit3/exhibit-api.js`.

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
Use Selmer to render part of `index.html`

Currently, Selmer is used to render the paragraph which states the
academic year and semester the application is currently at.

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
Use Python script for downloading api-nusmods files

And change the directory structure to under `api-nusmods-stuff` to
`YYYY-YYYY/semX`, where `YYYY-YYYY` is the academic year and `X` is the
semester. For instance, 2013-2014/sem2

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
Store `modinfo.js` and `auxmodinfo.js` at resources/public/js/mods/YYYY-YYY/semX

To support multiple semester timetables in the future.

Changes in this patch include:
- Modifying the gulp `generate-module-data` task to generate module
  information for academic year 2013-2014 semester 2. This is a big
  restriction but one which will be corrected in the future
- Changing `helpers/normalize_modules_json.js` to output the
  `processed_modules.json` file to the `api-nusmods-stuff/YYYY-YYYY/semX`
  folder. This drops the use of the `build-temp` folder for storing
  temporary build products
- Teaching `helpers/convert_modules.js` to write the `modinfo.js` and
  `auxmodinfo.js` directly to the
  `resources/public/js/mods/YYYY-YYYY/semX` folder. The folder is
  created using the `mkdirp` npm package
- At `resources/public/index.html`, use Selmer to interpolate the
  `acad-year` and `sem` values for the path to `modinfo.js`
- Changing cljs-nusmods.init to accept 2 parameters - `acad-year` and
  `sem`. This allows us to construct the path to the `auxmodinfo.js`
  folder for delayed downloading

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
README - credit Compojure and Selmer

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
[BUGFIX] - Append Select2 box to #timetable-builder-controls

instead of prepending. This is observed when we switch tabs.

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>