PARQUET-87: Add API for projection pushdown on the cascading scheme level

JIRA: https://issues.apache.org/jira/browse/PARQUET-87
Previously, the projection pushdown configuration is global, and not bind to a specific tap.
After adding this API, projection pushdown can be done more "naturally", which may benefit scalding. The code that uses this API would look like:

```
Scheme sourceScheme = new ParquetScroogeScheme(new Config().withProjection(projectionFilter));
 Tap source = new Hfs(sourceScheme, PARQUET_PATH);
```