Serial entrepreneur Mihael Mikek came to Boston from Slovenia to earn his graduate degree at Babson College. He founded Celtra, Inc., which develops software to create HTML5 ads for mobile devices. Mikek cites the U.S. as the ideal place to be an entrepreneur and grow a global business. Celtra had more than $160 million of estimated media delivery in 2014 and works with more than 600 certified media partners and more than 400 global clients.