Using vim plugin file layout

Vim users typically use a package manager to install plugins, instead of manually copying files. Here, the syntax file and stan file type detection are moved into their appropriate local paths. A user using the package manager Vundle, for instance, can then add
```
Plugin 'maverickg/stan.vim'
```
to their .vimrc to install your plugin. 

Ref:
http://robots.thoughtbot.com/writing-vim-syntax-plugins