Consider adding some icon helpers

This is something I've been considering for a while. I've been copying some font-awesome view helpers between some rails projects, but it'd be nice to have them consolidated. I've also been on a kick to consolidate the [competing](https://github.com/littlebtc/font-awesome-sass-rails) [font-awesome asset](https://github.com/balexand/font_awesome) [pipeline gems](https://github.com/kristianmandrup/font_awesome-sass-rails) and try to make this gem the canonical one (several others are outdated, rely on sass unnecessarily, etc). [One of the competing gems](https://github.com/kristianmandrup/font_awesome-sass-rails) had some helpers, which rekindled the idea.

In trying to formalize this proposal, I was thinking of two new helper methods, `fa_icon` and `fa_stacked_icon`. Before I dive into an implementation, I wanted to present some method signature examples and start a discussion.

```ruby
# <i class="icon-camera-retro"></i>
fa_icon "camera-retro"                                           

# <i class="icon-camera-retro"></i> Take a photo
fa_icon "camera-retro", text: "Take a photo"

# <i class="icon-camera-retro icon-large"></i>
fa_icon "camera-retro large"
# <i class="icon-camera-retro icon-2x"></i>
fa_icon "camera-retro 2x"
# <i class="icon-camera-retro icon-3x"></i>
fa_icon "camera-retro 3x"
# <i class="icon-camera-retro icon-4x"></i>
fa_icon ["camera-retro", "4x"]

# <i class="icon-quote-left icon-4x icon-muted pull-left"></i>
fa_icon "quote-left 4x muted", class: "pull-left"
# <i class="icon-flag icon-4x icon-border pull-left"></i>
fa_icon "flag 4x border", class: "pull-left"

# <i class="icon-refresh icon-spin"></i>
fa_icon "refresh spin"
fa_icon ["refresh", "spin"]

# <i class="icon-spinner icon-spin icon-large">
fa_icon "spinner spin large"

# <i class="icon-fixed-width icon-pencil">
fa_icon "pencil fixed-width"

# <li><i class="icon-li icon-ok"></i> Bulleted list item</li>
content_tag(:li, fa_icon("ok li", text: "Bulleted list item"))

# <i class="icon-shield icon-rotate-180">
fa_icon "shield rotate-180"
# <i class="icon-shield icon-flip-vertical"></i>
fa_icon "shield flip-vertical"

# <span class="icon-stack">
#   <i class="icon-check-empty icon-stack-base"></i>
#   <i class="icon-twitter"></i>
# </span>
fa_stacked_icon "check-empty", "twitter"

# <span class="icon-stack">
#   <i class="icon-sign-blank icon-stack-base"></i>
#   <i class="icon-terminal icon-light"></i>
# </span>
fa_stacked_icon "sign-blank", "terminal light"
```

**Update:** `fa_stacked_icon` signature changed. It's now:

```ruby
# <span class="icon-stack">
#   <i class="icon-check-empty icon-stack-base"></i>
#   <i class="icon-twitter"></i>
# </span>
fa_stacked_icon "twitter", base: "check-empty"
```