File system river

Hello there,

on the past week I was trying to use Lucene to automatically index a few text files inside a given directory on the file system. Without much success and with a change of requirements I then took a better look on elasticsearch and read about the river concept.

Today I created this river that monitors a given folder on the file system, and automatically indexes/remove files added/updated or deleted. It, in fact, worked quietly nice :D

There is one problem though, that is when I shutdown the server and start it up again. Seems like the river stops to work. Could you please advise me about it?

And later, if it's of your interest, accepted this pull request?

Thanks in advance.


ps.: here is a brief manual on how to use it:

After installing the plugin and running the server, create a river with:

```bash

curl -XPUT localhost:9200/_river/my_fs_river/_meta -d '
{
    "type" : "fs",
    "fs" : {
        "path" : "/home/user/tmp/files/",
        "check_interval" : "10000"
    }
}'
```

By the way, it expects the dir to be already created. In this example that's in /home/user/tmp/files/

If you know create a file, like this:

```bash

cat <<EOF > /home/user/tmp/files/
hello world
EOF
```
... in 10 seconds or less (based on the check_interval), the file will be indexed. I know java 7 has better ways to monitor for file changes, but since that's not very well adopted I preferred to not use those features. You can check that the file is there with:

```bash

curl -XGET localhost:9200/my_fs_river/status/a.txt
```

And, finally, search for the term:

```bash

curl -XGET http://localhost:9200/my_fs_river/status/_search -d '{
    "query" : {
        "term" : { "contents": "hello" }
    }
}'
```