Fix nested comments

The whole of the following program is highlighted as a comment for me in vim 7.4 (Ubuntu 14.04):

```kotlin
/* Hello, world! */

fun main(args: Array<String>) {
  println("Hello, world!")
}
```

This patch fixes that. The problem appears to be that `/*` is matched as the start of the inner/contained region rather than the start of the outer/containing region i.e. this works:

```kotlin
/* */ */

fun main(args: Array<String>) {
  println("Hello, world!")
}
```

The [documentation](http://vimdoc.sourceforge.net/htmldoc/usr_44.html#44.7) [doesn't help much](https://github.com/lambdatoast/elm.vim/pull/4), but using a `matchgroup` (and **not giving it the same name as the region**) seems to solve the problem, and even handles [`/*/**/*/`](http://vim.1045645.n5.nabble.com/Syntax-region-problem-td1164748.html) and [`/* */*`](https://github.com/kballard/rust/blob/54a4d0935f77291d680416269211ba7844cdc8bc/src/etc/vim/syntax/rust.vim#L181) ([via](https://github.com/rust-lang/rust/issues/12307)) correctly.

I also updated the README to include Vundle and Pathogen installation instructions.