Change pagenum warnings to INFO

[rtonsing]

WARNING:root:48 elements having class pagenum have been rewritten.

I suggest these be made just INFO. WARNING implies there is a problem, in this case there is none as far as the HTML is concerned, it is just that ebookmaker rewrites them for epub.

As a side issue, is there a way to NOT repeat warnings, etc. 3 times? I am a fan of keeping error output concise and helpful.

[eshellman]

Yes, these could be INFO messages. They predate my work on Ebookmaker.

Because of the way Ebookmaker makes a new job for each output type, removing duplicate warnings across jobs is really hard. I would really like to do some refactoring that would make it easier, but it's not looking like that will make it into v0.13. The duplicate warnings for the mobi outputs may go away in v0.13, because I need to do related work for PDF output.