Added the ability to specify a new size in px

Added the ability to specify a new size in pixels when you save out the files. Handy when exporting logos or some other variable-size graphic.
User defined prefix to seperate sizes

Added the ability for the user to define a prefix to seperate sizes.
ex: 16x16_logo1.png / 16x16_logo1@2x.png VS  64x64_logo1.png /
64x64_logo1@2x.png
Took prefix out of 4iOS.jsx, added sizing to WithPrefix
Merge pull request #1 from philiplikens/patch-1

Added the ability to specify a new size in px