Added `exists`.

I'd like to add [exists](http://nodejs.org/api/fs.html#fs_fs_exists_path_callback) to the `Async` version as well. However, this is one of the few(only?) function that actually doesn't throw an error as its first argument. So, implementing it means we have a couple of options:

1. Wrap it into a function that does nothing with its first `Error` argument.
1. Drop into the ffi and handle this one case.

The first one seems like a mismatch between the underlying api and this version. 
The second one seems a bit unnecessary of a thing to have to do for one specific function, but it brought up some questions in IRC. In specific, how can this function remove the effects safely?: https://github.com/purescript-contrib/purescript-node-fs/blob/master/src/Node/FS/Async.purs#L43-L52 I might be just missing somehting, but it doesn't seem safe.

I don't really care which way is prefered. I really only want the `Sync` version anyway. So, just let me know which to go with and I'll implement it.