Minor updates to documentation

[bro1]

Hi,

I have started using your library go-wordpress for my own hobby project, but I have encountered a couple of issues reading your documentation and finding the relevant WordPress plugins.

So this pull request is to update the documentation that would have saved me an hour or 2 :) - there are just so many WordPress plugins with similar names.

This also addresses the issue #2

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Functional fixes:

• Fix upload of media. WordPress and/or Apache were complaining about the Content-Disposition header being malformed.
• Fix setting the FeaturedImage for a post. According to http://v2.wp-api.org/reference/posts/ it's supposed json attribute is "featured_media" not "featured_image"

thanks