Let `resources` accept absolute path.

As mentioned in Kapeli/cheatsheets#31, I'd like to specify an absolute path for `resources` method but was not possible. This patch makes it possible to assign absolute path for `resources`.

Example (assume that Kapeli/cheatsheets repository is checked out in `projects/cheatsheets` dir):

```rb
# projects/cheatsheets/cheatsheets/My_Cheatsheet.rb
RES_DIR = `File.expand_path(__FILE__, "../resources/My_Cheatsheet/")`.
resources RES_DIR #=> resolves to /path/to/projects/cheatsheets/resources/My_Cheatsheet/"
```

If a non-absolute path is passed in, it will resolve relative path. For example:

```rb
# projects/cheatsheets/cheatsheets/My_Cheatsheet.rb
resources "assets" #=> resolves to '/path/to/projects/cheatsheets/cheatsheets/assets/'
```