Added automatic mime-type detection, so that also svg-files are displaye...

[grst]

...d properly.

Added note to README, that also local images are accepted.

[GSI]

I like the basic idea. Would you mind resolving the following two issues in your code:

• The URL is now opened twice (open()) -> Unnecessary overhead
• Your PR contains several garbage files (backups of the gemspec file)

[grst]

done.

[GSI]

Nice and shiny. Thank you.