Show "Nothing found" text when search fails.

That's much better than the previous flickering where before performing
the search the dropdown was shown and after getting no results it was
hidden again.

Better to just show a message "Nothing found" - this will also make
clear to the user that the search found no results and didn't just fail
for whatever reason.
Allow Markdown file to be used with --welcome.

Added additional styles for the welcome page so that when it's
rendered from Markdown, the default result will look OK.
Document ECMAScript 5 method in builtin classes.
Up version to 4.6.1
Warn when referencing missing images from guides.

Achieving this required extensive refactoring of how the images get
processed.  Previously the existance of images was looked up after
all the @img tag had been replaced.  Now the check for existance is
part of expanding the @img tag, so the check happens also for images
in guides, for which the check didn't occour previously as there is
no images copy step in there.

The old Images class has been replaced with ImageDir and ImageDirSet.
Add width and height attributes to expanded @img tags.

Include the Dimensions gem to retrieve image width/height meta-data.

Additional %w an %h placeholders to --img-tpl template.
Report unused images in guides.

Both the ImagesDir and ImagesDirSet now have #report_unused method.
No more is the unused images reporting part of #copy method.

Also take care to ignore the icon.png and icon-lg.png files inside
guides - these don't have to be reference by @img tag, but are
used automatically.
Move ImageDir and ImageDirSet to Img:: namespace.
Extract Img::Writer class from Img::DirSet.
Extract #format_guide method from Guides#load_guide.

Just an ordinary refactoring.
Merge branch 'master' into doc-tags
Process @param tag subproperties inside Tag::Param.
Print subproperty warnings inside Tag::Param.

To achieve this, all #process_doc method get a third argument - the
position hash {:filename, :linenr}.

I considered several other options for passing this information
to the #process_doc method, but adding an instance variable for
all the tag classes didn't seem like a right way to go.
Process @return subproperties inside Tag::Return.

The :return field in the final hash will now be nil when no @return
tags was encountered - no more will it default to "undefined" type.
Remove CSS mixin name detection from DocAst.

There is no @tag for CSS Mixins, and therefore no way to define
a mixin name inside documentation - it can only be derived from code.
Process @property tag fully inside Tag::Property.

Additionally process the @type tag.
Process @cfg tag fully inside Tag::Cfg.

Now we have completely removed the use of Doc::Subproperties from
Doc::Ast class.
Process @var tag fully inside Tag:CssVar.
Move warning logging back to Doc::Subproperties.

Gets rid of all the duplicated warning logging code from the Tag classes
that use Doc::Subproperties.
Refactor away extroneous code from Doc::Ast.

Specific methods per each member type are no more needed - we can cover
it all with just a little bit of generic code.