Enforce terms consistency for Pin it/Pinit

[deshack]

As a rule of thumbs, Pinit is the plugin name, whereas Pin it is the name of the Pinterest Pin it button.

In the readme we found something like this:

<p>Pinit Button for Japanese Language</p>

Here Pin it should be used instead of Pinit.

I'll take care of looking for all the occurrences of both words and ensure they're consistent throughout the whole project.

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