introduce a caching layer for s3mpi

This introduces a caching layer for s3mpi that will keep copies of large files on the disk. There is no support yet for pruning of old objects in the cache or selective non-use of the cache, so be careful. You can enable it by adding

```r
options(s3mpi.cache = '~/.s3cache')
```

to your `~/.Rprofile`