Tracevis improvements

@yborovikov @jodzga 

    Trace improvements

    The biggest change that comes with this commit is that we now track how
    long it takes to execute a Task's run method.

    This change also includes some tracevis improvements:

    - Table view
        - Show time with microsecond resolution.
        - Show the end time in a column.
        - Show the time it takes to transition to invoke a Task's run
          method.
        - Use a monospace font and align all times in each of the numeric
          columns.
    - Graphviz view
        - Show the time it takes to transition to invoke a Task's run
          method.
        - Align all times for the numeric cells.
    - Waterfall view
        - Show the time it takes to invoke a Task's run method using a
          darker color.
        - Color scheme improvements.
        - Colors are managed through CSS instead of JS.