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);
```

Author: Tianshuo Deng <tdeng@twitter.com>

Closes #51 from tsdeng/projection_from_scheme and squashes the following commits:

2c72757 [Tianshuo Deng] make config class final
813dc1a [Tianshuo Deng] erge branch 'master' into projection_from_scheme
b587b79 [Tianshuo Deng] make constructor of Config private, fix format
3aa7dd2 [Tianshuo Deng] remove builder
9348266 [Tianshuo Deng] use builder()
7c91869 [Tianshuo Deng] make fields of Config private, create builder method for Config
5fdc881 [Tianshuo Deng] builder for setting projection pushdown and predicate pushdown
a47f271 [Tianshuo Deng] immutable
3d514b1 [Tianshuo Deng] done