No home link on "page" layout

[oAGoulart]

Issue

Pages with the page layout have no navigation header. User cannot go to home page or main pages after clicking on a post.

Proposal

1. Move navigational header from index.html to default layout.
   

   textlog/index.html

   Lines 5 to 9 in 9ae8898

   	<section class="site-header">
   	<h1 class="smallcap"><a class="site-title" href="{{ '/' | prepend: site.baseurl | prepend: site.url }}">{{ site.title }}</a></h1>
   	{% include nav.html %}
   	{% include intro.html %}
   	</section>

2. Remove headers from index.html, categories.html, tagged.html. Also, use page.description with the modification above. As an example, this:
   

   textlog/categories.html

   Line 9 in 9ae8898

   <p>All categories posts will be sorted here.</p>

   
   ...would become this (allow each page to set its description on the front matter):

   <p>{{ page.description }}</p>