fix import-posts pass through 'since' as nil

adds a conditional to the invocation of import-post in import-posts loop

there needs to be a guard around 'since' in the loop:
if import posts tries to pass it's optional 'since' parameter through to
import-post it will prevent any posts from being output , because every
post will be compared against a timestamp of nil
Merge pull request #63 from cmstrickland/wp-import-since

optional 'since' handling in WP import plugin