<rdar://problem/14134716>

This is a rewrite of the command history facility of LLDB

It takes the history management out of the CommandInterpreter into its own CommandHistory class
It reimplements the command history command to allow more combinations of options to work correctly (e.g. com hist -c 1 -s 5)
It adds a new --wipe (-w) option to command history to allow clearing the history on demand
It extends the lldbtest runCmd: and expect: methods to allow adding commands to history if need be
It adds a test case for the reimplemented facility