Remove the "Some Title" from pages generated

This is a little hack. Assuming that every IPython Notebook start
with a header `h1` we don't need the title field in the Markdown
template.

Why it is a hack?

When converting a Markdown file the HTML will have something like

~~~
<div class="span10 offset1">
    <h1>
        Files and Directories
    </h1>
~~~

When converting the IPython Notebook to Markdown and them to HTML
will have something like

~~~
<div class="span10 offset1">
    <h1></h1>
    <h1 id="analyzing-patient-data">
        Analyzing Patient Data
    </h1>
~~~

Firefox and Chrome won't add any space to the first `h1` tag and
for the user will look like that all the files have a more-or-less
uniform appearance.
Avoid empty 'h1' tag

Change in lesson layout to avoid empty 'h1' tag when build lessons
from IPython Notebook using *.ipynb -> *.md -> *.html workflow.
Merging cell-tag-preserving template change proposed by @r-gaia-cs
Getting rid of _files directories
Merge branch 'fix-title' of github.com:r-gaia-cs/bc into fix-title