History Timestamps for Ion

Adds the ability to record timestamps for commands in the history file. The time stamps are recorded in a raw number format as mmstick requested, for easy parsing. By default this feature is disabled and can be enabled by changing the value of the environment variable HISTORY_TIMESTAMP from 0 to 1. This merge request was done for the issue #715 (closed) listed by mmstick. The timestamps are listed in a bash style format, where the timestamps is denoted by a # character and its corresponding command is listed on the following line. A simple way to handle timestamps.