Allow string input for Kss::Parser

Allowing string input (with `Kss::Parser.new(string)`) allows dynamic use cases in live web applications

Example:

```ruby
buttons =<<-'EOS'
  /*
  Your standard form button.

  :hover    - Highlights when hovering.
  :disabled - Dims the button when disabled.

  Styleguide 1.1
  */
  button {
    padding: 5px 15px;
    line-height: normal;
    font-family: "Helvetica Neue", Helvetica;
    font-size: 12px;
    font-weight: bold;
    color: #666;
    text-shadow: 0 1px rgba(255, 255, 255, 0.9);
    border-radius: 3px;
    border: 1px solid #ddd;
    border-bottom-color: #bbb;
    background: #f5f5f5;
    filter: progid:DXImageTransform.Microsoft.gradient(GradientType=0, startColorstr='$start', endColorstr='$end');
    background: -webkit-gradient(linear, left top, left bottom, from(#f5f5f5), to(#e5e5e5));
    background: -moz-linear-gradient(top, #f5f5f5, #e5e5e5);
    box-shadow: 0 1px 4px rgba(0, 0, 0, 0.15);
    cursor: pointer;
  }
  button:disabled {
    opacity: 0.5;
  }
EOS
@styleguide = Kss::Parser.new(buttons)
```

We can then imagine a web app for prototyping designs or html components that would provide live kss documentation & demo rendering without relying on static files.
Add string use case in the example app
Add string use case in the README.md
Document the CommentParser#parse_blocks method