Image tag policy

Closes #93 

Related to https://github.com/Ink/filepicker-rails/pull/95 https://github.com/Ink/filepicker-rails/pull/104

@mhoran @dannysullivan can you guys take a look :heart: ? The argument for `expiry`, need to be a callable, I added this [here](https://github.com/Ink/filepicker-rails/commit/35df298ea9651ef20b6018313befee4283029da4).

I open this PR, to make easy to see the changes. If it is ok to you guys I will merge :smile: