ENH: TextBox stim

* Added 2 methods to TextBox class:
    + getTextGridCellPlacement(): return a numpy array giving the pos, width,
      and height of each textgrid cell inthe Textbox.
    + getGlyphPositionForTextIndex(i): return the bounding box for the text
       character at index i within the text displayed. This is != to the
       textgrid cell the char is in, instead is the bounding box of the draw
       glyph itself.

* Changed examples:

    - simple_textbox: straight forward TextBox use, showing 2 different uses
       of common params.
    - textbox_glypth_placement: displays the mouse cursor with the current
       mouse x,y window position (in monitor units), useful for checking the
       accuracy of the information returned by getGlyphPositionForTextIndex()
   - textstim_vs_textbox.py: test different timing conditions for the
       TextBox and TextStim, allowing a user to check what performance (and
       differences) to expect with the two text stim types with a string of a
       specified length.