Adding the ability to start/stop the TicTocTimer

[jsiirola]

Fixes: N/A

Summary/Motivation:

This adds the ability to start/stop a TicTicTimer. If the timer was stopped and restarted, then the logged message will also include the number of times the timer was explicitly started.

Changes proposed in this PR:

• Add TicTicTimer.start() and TicTicTimer.stop()
• Absolute time is now counted from when the TicTocTimer was instantiated (not the time the module was imported).

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1. I agree my contributions are submitted under the BSD license.
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[coveralls]

Coverage decreased (-0.2%) to 62.127% when pulling ba9fd592b75badc0d06a33fc3bc8b4283ac82ddd on timer-start-stop into 3f67b2653329d272d5c1199afe0eec4b13022c9e on master.

[jsiirola]

@michaelbynum: Yes. You can use toc(). It returns the time it prints, and if you set up the timer to be a cumulative timer, then the time that is returned is the current cumulative time. If you want to get the time without actually printing anything, then toc("") will do what you want.