Add Helvetica font family

[mpospese]

Add a Helvetica font family, patterned off of the HelveticaNeueFontFamily, only it loads the Helvetica font and not the Helvetica Neue font. It should handle regular and italics and cover all weights that are installed on iOS devices.

You can query the iOS simulator to get a list of all installed fonts to see which weights of Helvetica are installed and what they are named.

(Eventually we should create FontFamily implementations for every preinstalled iOS font, e.g. AvenirNext, etc.)