Added new FrameworkFeature class + missing exception:

Added new FrameworkFeature class + missing exception:
- Features can be enabled or disabled by the user
- Their availability depends on if the feature is enabled and if all
  requirements are found. This can be anything like loaded PHP extensions (PECL)
  or anything else.
- Features can have methods that can be called (publicly) where their actual
  "feature" is being written.
- This can be again a class with static or non-static methods that will be
  called.
- Generally feature classes shall not contain the actual implementation, they
  shall only call implementing classes.
- So basicly a feature class "hides" the actual implementation and can be
  understood as a generic facade.

Signed-off-by: Roland Häder <roland@mxchange.org>