[with patch, positive review] clean up how the percent directives at the top of cells are processed

[jason]

Notes from Mike looking at this:

• Docs for parse_percent_directives needs to give what is returned
• is_html doesn't need to call parse_percent_directives
• url_to_self should have at least one line of documentation besides the examples
• %hide and %hideall can also be cleaned up from the html and html_in methods (maybe other places)

[jason]

One positive review. It's probably important enough that at least one other person ought to review it as well.

Mike and I stepped through the changes, they all look good, and I tested for about 20 minutes with various combinations of the directives and things seemed to work. I had one glitch, but I can't reproduce it. I had a cell that looked like:

%hide
%hideall
print "hi"

I closed the cell, reopened it, then deleted all output, then did a few more things I can't remember. At one point, the cell changed to:

%hide
%hid

If we can't reproduce something like this, though, I give it a positive review.

[jason]

In the above glitch, I mean to say that I closed the *worksheet*, reopened it, etc...

[rlm]

Applies cleanly to Sage 3.3.alpha0, passes doctests, and seems to be working when I test it out. This is on Intel OSX 10.5.

[mabshoff]

Merged in Sage 3.3.alpha1