Implementation of the blog generator

This commit adds implementation of the blog generator, which
incorporates feed, index page and tags pages generators.
Please, make some indentation when you continue param description in a new line.

I mean:

```
:param documents: a list of Document objects, generated by Document
                  class from files in the content directory
```

or

```
:param documents: a list of Document objects, generated by Document
    class from files in the content directory
```

but not

```
:param documents: a list of Document objects, generated by Document
class from files in the content directory
```

It's about readability. :)