Adds AGU journals template

[eliocamp]

This adds a template for the AGU journals. Tested the skeleton on my machine and it worked. I hope everything else is in order.
Haven't signed up the Contributor Agreement yet because I don't have access to a printer right now, but will do next week :).

[eliocamp]

I've signed and sent the agreement. :)

[matthewross07]

Many thanks @eliocamp , I just pulled this branch for an upcoming submission to WRR. Hope to see this request merged soon.

[yihui]

Are the files under agu_article/skeleton/trackchanges-0.7.0/ necessary? There are a lot of them.

[eliocamp]

They come with the template and is used by the journal to track changes during the submission process, I think.
The template works perfectly without them.

[yihui]

I see. I'd prefer not to have them in the rticles package. You could add instructions in skeleton.Rmd for those who want to use this feature.

[eliocamp]

Ok! I'll be uncomputered until march, I'll do it then!

[yihui]

No problem. Thank you!

[eliocamp]

I've removed the trackchanges files and added instructions for using it.

[yihui]

I'll change the option name options to classoption and merge this PR. Thank you!