Four entrepreneurs in Gaza have an idea born out of deep need: A piezoelectric device that charges your mobile phone while you walk. In Gaza, say Omar Badawi, Sameer Alnunu, Ghaidaa Hussein, and Saeda Nassar, people cope with about four hours of electricity a day after last year’s war between Hamas and Israel. Gazans’ mobiles are lifelines to the outside world, which means keeping them charged is a critical need…