Fixes #240 and #216 Retry mongodb connection

I just fixed that for me (mongodb 2.4 and ES 1.2) on top of the 2.0.1 release, but if it can help other people!

- Retry if mongodb is down while river init till mongodb comes up
- Retry if mongodb isn't reachable while processing the oplog

Can be install directly 

```
plugin --install com.synhaptein/elasticsearch-river-mongodb/2.0.1.1
```