Update sailthru.rb

Added preview commands

preview_Template_With_HTML(template, email)
preview_Blast_With_HTML(blast_id, email)
preview_Recurring_Blast_With_HTML(blast_repeat_id, email)
preview_Content_With_HTML(content_html, email)
preview_Template_With_Email(template, send_email)
preview_Blast_With_Email(blast_id, send_email)
preview_Recurring_Blast_With_Email(blast_repeat_id, send_email)
preview_Content_With_Email(content_html, send_email)