HFile tool should implement Tool, disable blockcache by default

I tried using bin/hbase hfile in a memory-constrained environment. It crashed while trying to instantiate a blockcache. Went to override the configuration and found I couldn't.

Refactor HFile to remove main(), put implementation entirely in HFilePrettyPrinter. Said class now extends Configured and implements Tool so configs can be overridden on cli. Also disabled blockcache by default.