Add basic LRU cache for reading footers.  Need some more input on what size the cache should be set at (or how to make it configurable)
Add basic LRU cache for metadata.  This cache also checks the modification times of the underlying files to determine whether the metadata needs to be re-read from disk or not.
Conflicts:
	parquet-hadoop/src/main/java/parquet/hadoop/ParquetFileReader.java

Conflicts:
	parquet-hadoop/src/main/java/parquet/hadoop/ParquetFileReader.java