Removed semi-colons from command example, as DEBUG and TUMBLR_API_KEY
env-vars will *not* be in the environment otherwise and api_key will not be set.
Moved url format string to inner loop, as otherwise offset is re-initialised to 0 every iteration. Without this, the first 20 posts are re-appended for (n) steps.
Merge pull request #3 from squidsoup/readme-fix

Removed semi-colons from command example
Merge pull request #4 from squidsoup/url-format

Moved url format string to inner loop