Improve markup with more semantic HTML tags

Fixes #405
Add rel=author support in blog and profile pages

* add G+ info in member profile
* add rel=author on links to author page (in blog posts)
* add rel=me on links to external profiles (in profile page)

Fixes #404
Add Twitter cards support in blog posts

Add meta tags to support Twitter summary cards in blog posts.

Fixes #436
Use opengraph microformat in site pages

This commit adds opengraph metadata in pages, with at least:
* og:title
* og:description
* og:image

Fixes #403
Fix Thymeleaf expression in understanding pages
Use Jackson for JSON and XML serialization

Prior to this commit, multiple ObjectMapper instances were created
in the application and org.simpleframework.xml was used for XML
deserialization.

This commit:
* creates a single ObjectMapper bean and inject it in services and
controllers
* uses jackson XML support for XML serialization

Fixes #439
Fix STS/GGTS download shortcut links

As of STS/GGTS 3.6.0, native installers aren't provided anymore.
So we should favor *.zip and *.tar.gz downloads (depending on the
targeted platform).

Fixes #442
Add links for Eclipse 4.4-based update site to download page

Fixes #443
Fix integration tests with STS/GGTS download changes

Add fake MILESTONE releases to the test XML download file.
See #442 and #443
Merge branch 'seo'

Conflicts:
	sagan-site/src/main/resources/templates/tools/ggts/all.html