added Project Euler solutions (15, 17, 131, 135, 137, 138, 139, 140, 141), and 

added a print out of total time elapsed (last line).
Total time is about 1 second, with 139 commented out (adding it back in would add about 2.5 seconds - is that too slow?)

I'm still looking for a nice "issquare" function - anyone has one?