Fix processing of multiple 'to_name' unicode objects.

Currently, the method sendgrid.message.Mail.add_to_name only encodes singular
unicode object 'to_name' names to Python str objects. Lists of multiple unicode
objects passed into the aforementioned method does not elicit the same encoding
process. This commit fixes that.
Merge pull request #87 from cbg/fix_unicode_to_names

Fix processing of multiple 'to_name' unicode objects.