PARQUET-4: Use LRU caching for footers in ParquetInputFormat.

Reopening https://github.com/Parquet/parquet-mr/pull/403 against the new Apache repository.

Author: Matthieu Martin <ma.tt.b.ma.rt.in+parquet@gmail.com>

Closes #2 from matt-martin/master and squashes the following commits:

99bb5a3 [Matthieu Martin] Minor javadoc and whitespace changes. Also added the FileStatusWrapper class to ParquetInputFormat to make sure that the debugging log statements print out meaningful paths.
250a398 [Matthieu Martin] Be less aggressive about checking whether the underlying file has been appended to/overwritten/deleted in order to minimize the number of namenode interactions.
d946445 [Matthieu Martin] Add javadocs to parquet.hadoop.LruCache.  Rename cache "entries" as cache "values" to avoid confusion with java.util.Map.Entry (which contains key value pairs whereas our old "entries" really only refer to the values).
a363622 [Matthieu Martin] Use LRU caching for footers in ParquetInputFormat.