Pimping up emCalc

Fixes https://github.com/zurb/foundation/issues/2615

call emCalc with just values (10)

		.body{
		margin: emCalc(10 20 30 40);
		}
		
or values with units (10px)
		
		.body{
		margin: emCalc(10px 20px 30px 40px);
		}
		
returns 
		
		.body {
		  margin: 0.625em 1.25em 1.875em 2.5em;
		}

use any number of parameters

		.body{margin: emCalc(10px)}
		.body{margin: emCalc(10px 20px)}
		.body{margin: emCalc(10px 20px 30px)}
		.body{margin: emCalc(10px 20px 30px 40px)}

Use multiple paremeters without units 

		.body{margin: emCalc(10)}
		.body{margin: emCalc(10 20)}
		.body{margin: emCalc(10 20 30)}
		.body{margin: emCalc(10 20 30 40)}

Mix and match 

		.body{
		margin: emCalc(10px 30 40px);
		}