Created parent dir for all static doc assets

The CSS and images embedded in the documentation HTML output now share
a common parent directory: docs/static/.  I need to add a copy of
FontAwesome to the static asset set, and I don't want to have to
modify the pom every time I add something like this, so I'm creating a
single parent directory to ease future static asset additions.

This commit also includes a bunch of titanblob.css style changes.
Adding FontAwesome to support Titan docs

This is needed to render the little up-arrow in the
jump-to-top-of-page button that renders at the lower-right-hand side
of the viewport and travels with the viewport as you scroll down the
page.  I can't use the FontAwesome copy on the Titan prototype site
because of XSS import restrictions.
Adding new styles Titan doc HTML

Styling code highlights, tables, figures, and the page background
color
AsciiDoc markup tweaks

Pulled all headings up one level.  The top level is now the Part
instead of the Chapter.  The Part is the highest level available, so
relatively speaking, this just opens up a new level of organization at
the bottom of the hierarchy that we previously didn't have.

Cleaned up some wikidoc [[pagename]] style links, turning them into
_TODO: xref to pagename_ where the page hasn't yet been ported from
the wiki.

Made the HBase and Cassandra logos float left on the benefits-of-titan
page where they are featured.

Added [source,gremlin] annotations to Gremlin output in preparation
for creating a custom xslthl highlighter that can parse Gremlin.  Also
tried [source,bourne], but the default output for shell scripts is so
underwhelming (only "cd" was highlighted and everything else was in
plain font) that I decided I would rather forego it.

Fixed a misformatted numbered list in intro.txt.
Adding a small Cassandra logo SVG from Wikipedia
Improving DocBook code highlighting, style tweaks

Added a primitive highlighter for Gremlin that just highlights the
"gremlin>" prompt, the return value prefix "==>", string literals, and
comments.  In a future revision, I would like to teach the highlighter
to recognize Gremlin language functions like g.V and map, but this is
a start.

This config includes a local file:/// path that's only valid on my
system (in titansingle.xsl).  This is due to a limitation in xslthl;
it needs to be told where its config file is, but it can only process
either http:// or absolute file:// values, not relative paths.  I'm
probably going to end up filtering that stylesheet through
maven-resources-plugin so that I can substitute an absolute path based
on Maven's ${project.basedir} property.

This commit also removes the old asciicommon.xsl import held over from
AsciiDoc.  I gathered the settings relevant for our current
DocBook->HTML output into titansingle.xsl and deleted asciicommon.xsl.

I also made some minor changes to the style of the HTML output
produced by DocBook.
Enable Maven property filtering on XSL stylesheets

$MAVEN{...} can now be used inside XSL stylesheet sources in docs/xsl
to substitute Maven pom property values, e.g. $MAVEN{project.basedir}.
This eliminates the need for a pair of file path hacks in
titanchunk.xsl and titansingle.xsl.
Trim some cruft from a source snippet in the docs

I added a superfluous block comment, class definition, and method
definition to one of the documentation's code snippets while getting
xslthl code highlighting running.  Removing these now that
highlighting is operational.
Fix broken intra-docs links

Converted some leftover "display name":pagename and [[pagename]] style
links to AsciiDoc style xrefs.  Left a TODO where the target page
hasn't been ported to AsciiDoc yet.
Only generate TOC at top of singlefile doc HTML

Generating a bunch of TOCs at the Part level per DocBook default is
kind of disorienting when reading the single-file HTML output.  The
new Titan style has a "jump-to-top-of-page" in the lower-right-hand
corner of the viewport at all times anyway, and that or the Home key
are probably handier for TOC navigation than inserting a bunch of
miniTOCs inline anyway.

TOC generation in the chunked output is unchanged.