Lazy initialize engine, so that it doesn't consume time if no document needs to be rendered
Integrate with OrientDB
   * make use of OrientDB to store documents converted by parsers
   * support for incremental rendering if source file changed
   * prepare work for multiple document types
   * prepare work for multiple template engines
Rendering architecture rewrite part 1
   * Introduced abstract renderer
   * Freemarker is an implementation of a renderer
   * Renderer delegates to FreemarkerRenderer
Rendering architecture rewrite part 2
    * Renamed renderers to template engines (closer to the concept)
    * introduced a delegating template engine
    * separated Freemarker from core and made it optional
    * tags do not require the full list of docs in RAM anymore
    * renamed org.jbake.parser.Engines.properties to MarkupEngines.properties
    * "index", "archive" and "feed" are normal document types (config options are now optional)
Rendering architecture: support for Groovy template engine
   * added GroovyTemplateEngine
   * GroovyRendererTest test case uses .gsp files equivalent to .ftl
Add support for custom document types
   * read from configuration file (template.[doctype].file)
Clear cache if templates have changed
    * template change <=> force rendering of all documents
    * use SHA1 based hashing to compute a checksum of the templates directory
    * use the directory granularity because templates may include other templates
Make Groovy dependency optional
Refactor renderDocument to use a Writer instead of File
   * Writer is more modular
   * allows the Groovy template engine to gain access to the other rendering engines
   * so you can do "include 'freemarker-template.ftl' from a Groovy template
Removed APL2 license
Introduce optional "cached=false" metadata
   * :jbake-cached: false inside Asciidoctor files
   * forces rendering of a page even if contents or template hasn't change
   * fix rendering of index
   * make all document types (including custom types) available from templates
Make "alltags" available to templates
Fix sitemap rendering using writer
Add published_pages to template engines
Merge pull request #68 from melix/rendering-modularization

Rendering modularization