Use ImageOptim to shrink the images
Allow forcing the status code to be 200 OK

Some clients can't handle status codes in the 5xx range (Flash…), so
allow forcing it to always be 200 OK.
Create a changelog
Some cleanup in endpoint.rb
Removing parse_twice, as I don't think it's needed

Since this generates a standalone image, we can't use the common
counters for figures, pages and so on, and so most, if not all, reasons
to parse the document twice is removed.
Remove LatexToPdf.config, as it didn't do anything
Note about cleanup
Move data to a helper to avoid instance variables, and passing it around
Release v0.3.0