Rich-Text/Redactor hyperlink classes
If you only want to style how the link looks visually, and need a work-around, you could use my Redactor Styles for Craft plugin. If you add an inline style to the plugins Styles JSON object like this:

[
  {
    "btnName":"Action button", 
    "className":"btn-action", 
    "spanClass":"btn-action"
  }
] 


Depending on which order the user creates the link and marks the text/link and select style, you now can get the following markup from your rich text field:

&lt;span class="btn-action"&gt;&lt;a href="http://loremipsum.net"&gt;A link&lt;/a&gt;&lt;/span&gt;
&lt;a href="http://loremipsum.net"&gt;&lt;span class="btn-action"&gt;A link&lt;/span&gt;&lt;/a&gt;


If this works for you or not depends on your design. Unless there are some styles on the  that can't be removed, you could style it so that it looks the same by using:

.btn-action &gt; a,
a &gt; .btn-action { ... }  


You could also instruct your users to always create the link, then select the style, or the other way around. But, it's of course a very fragile solution. 

Thinking of making a new plugin just for creating more advanced links, the built in one doesn't really cut it.