support multiple WHISPER_DIRS

This adds support to graphite-web for searching multiple WHISPER_DIR's by renaming the WHISPER_DIR setting to WHISPER_DIRS and to a list from a string.

Includes backwards compatibility for WHISPER_DIR setting.

example:
```
WHISPER_DIRS = ['/opt/graphite/storage/whisper/01', '/opt/graphite/storage/whisper/02']
```

I'm experimenting with a graphite install where each carbon-cache instance is configured to store its whisper DB's in its own directory to facilitate easier cluster scaling. Supporting multiple WHISPER_DIRS makes it easier to move instances to new servers, for example: `rsync -a /opt/graphite/storage/whisper/01/ newserver:/opt/graphite/storage/whisper/01/`.