Bugfix for custom Callout

[nikischin]

Fixes #56 and fixes #57

This bug fixes a behavior where React would throw an error on component unmount. Also it fixes an issue where the callout would be visible although not being clicked at the end of the page.

One thing I would like to challenge while merging this PR. For the implementation I had before I attached the annotation callouts within their html element in the page whereas I attached those of the marker to the document body. With this fix, I attached both annotations to the document body, however, I would like to challenge this behavior and would be interested in your opinion on this @Nicolapps