Fix alternate lang tags, x-default, translations, nullpointer on dev, documentation, locale-encoding

[leobardimpact]

• only include translations that exist,
• add rel=alternate,
• x-default should be neutral according to spec
• both in Meta and sitemap
• adding documentation to code with links to relevant standardization documents
• return standard-conforming hreflang encoding for tags for rel=alternate and locales for og:locale, removing .utf8 encodings and transforming "-" and "_" as necessary
• fixing nullpointer in sitemap reading App::plugin('tobimori/seo')->version() when installing branches during development

Based on these pull requests, incorporating them

• check if translation exist before adding alternate tags #107
• Add rel attribute to alternate urls #113

Fixes:

• Multi-lang alternate tags: missing rel="alternate" #95
• Remove x-default tag #96
• Multi language: Pages translated only for some languages #104

Maybe @FlorianBoe and @timnarr, could you review and confirm?