Transform links to .md files into links to .html files.

Related issue: #2
Add header and --title option to set text in it.

Style the header similarly to JSDuck docs header.

Also improve the styling of --index-generated menu.
Add footer to all pages and --footer option.

Almost exactly the same as in JSDuck docs.
Add Rakefile with tasks: gem & spec.
Add Rake task :bump.

Helps us to easily increment version number.
Up version to 0.0.6.
Minor copy-paste bug in :gem task.