Update 99problems.md

Hi, I've written solutions for the problems that do not already have one.
please have a look at this first one, just picked 8 queens for its simplicity
and let me know if any comments are required or indeed if changes 
should be made to the style of the code
once the issue of style is resolved I will put up all the
other ones fairly quickly     :))
I think O'Caml is a beautiful language and would like to help
others learn it with more solved problems availabe to them
with sincre regards
hossein berry