Script timer: Measure the time a script takes to execute

This class can be used to measure the time a script takes to execute.

It can take note of the time it takes reach each part of a script takes to run and displays the results in a HTML table, so it can be used to analyse which parts of the script may need optimization.

It features:

* Add time markers recorded before class instantiation
* Temporarily pause the timer
* Group markers by script
* Optionally add subtotals per script (group)
* Uses PHP bcmath functions if possible for better accuracy
* Adjustable precision for the calculations
* Adjustable precision for the output
* Provides the following measurements:
- run time for the marker (excluding processing by this class)
- run time for the marker as a percentage of the total run time
- cumulative run time up to the marker
- cumulative percentage
* Customizable presentation of the results returning:
- a bi-dimensional array for custom external result processing
- an HTML string for use with a template system
- HTML for presentation to the user without further processing
* The details of presentation of the results can be customized with CSS