use sparkle for rendering sparklines

I was getting a bit fed up with all the issues of trying to get inline images to work, and I like to avoid persistence whenever possible, so went back to my [original hunch](https://github.com/afeld/tricle/issues/29#issue-17888557) of generating them externally. It was shockingly simple, actually... I think I went from zero to deployed in about ten minutes :muscle: 

https://github.com/afeld/sparkle

A few big benefits:

* Tricle users won't have to worry about installing ImageMagick
* The sparklines will appear in MailView (and Mailcatcher, and the [previewer](http://api.rubyonrails.org/v4.1.0/classes/ActionMailer/Base.html#class-ActionMailer::Base-label-Previewing+emails) in Rails 4.1+)

Tested in GMail, and they work great. Uncached on a single Heroku dyno, they are generating in 40-100ms. For GMail, at least, doesn't even look like I need to serve it over SSL :boom: 

The net diff: https://github.com/afeld/tricle/compare/master...sparkle

/cc @ajb @ilyakava