tumblr_backup: fix image extension checking (fixes #31)

If a user backups a blog using a domain name while using the option
`-Ibi` then the condition testing for a file extension will always be
true because the domain name contains a period.
tumblr_backup: provide pagination for monthly pages (fixes #26)

Option -N defines the number of posts per page; the default is 50.

Original patch by @WyohKnott; thanks!
Merge branch 'gh-26'
tumblr_backup: add acknowledgments
tumblr_backup: fix Queue errors during Python interpreter shutdown

There goes my threadpool in less than 30 lines 😞
tumblr_backup: use the constructor to initialize things
Merge branch 'master' into api-v2