Add man pages and their sources

Hi,

it would be nice to add man pages for this gem. Since I package it for Fedora[1], I could then make sure, that users installing the RubyGem RPM will be able to use `man sdoc` to see the documantation and users using upstream gems could use `gem-man`[2] plugin to see it.

I used Asciidoc and added sources as well. The man-formatted files are then generated by:
```
a2x --doctype=manpage --format=manpage sdoc.1.asciidoc
```

I included only the basics so far, but I think it's a good start. Please consider.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=991339
[2] http://defunkt.io/gem-man/