Here is the Demo
If you use 2.3.1, uncomment line 12 and you will see the button appearing correctly
Does anyone know how can I fix it for version 3.0.0-rc1?
The answer below was true for Bootstrap 3 Release Candidates, however the stable version does include the Glyphicons library which requires a base class and an individual icon class, ex:
<span class="glyphicon glyphicon-calendar"></span>
From the docs:
With the launch of Bootstrap 3, icons have been moved to a separate repository. This keeps the primary project as lean as possible, makes it easier for folks to swap icon libraries, and makes Glyphicons icon fonts more readily available to more people outside Bootstrap.