HTML_Template_Sigma implements Integrated Templates API designed by Ulf Wendel.

Features:
- Nested blocks. Nesting is controlled by the engine.
- Ability to include files from within template: <!-- INCLUDE -->
- Automatic removal of empty blocks and unknown variables (methods to manually
  tweak/override this are also available)
- Methods for runtime addition and replacement of blocks in templates
- Ability to insert simple function calls into templates: func_uppercase('Hello
  world!') and to define callback functions for these
- 'Compiled' templates: the engine has to parse a template file using regular
  expressions to find all the blocks and variable placeholders. This is a very
  "expensive" operation and is an overkill to do on every page request:
  templates seldom change on production websites. Thus this feature: an internal
  representation of the template structure is saved into a file and this file
  gets loaded instead of the source one on subsequent requests (unless the
  source changes)
- PHPUnit-based tests to define correct behaviour
- Usage examples for most of the features are available, look in the docs/
  directory
