Add StateContext into StateMachineListener

- new StateMachineListener method for listening all
  StateContext changes in a various stages.
- StateContext now has a stage enum field telling at
  which stage it is.
- Polish some classes and docs.
- Prepare tests for StateContext.
- Relates to #126, #138 and #150

docs/src/reference/asciidoc/index.adoc

@@ -9,6 +9,7 @@
 :core-dao-exceptions: http://docs.spring.io/spring/docs/{spring-version}/spring-framework-reference/html/dao.html#dao-exceptions
 :core-jdbc: http://docs.spring.io/spring/docs/{spring-version}/spring-framework-reference/html/jdbc.html
 :core-jdbc-JdbcTemplate: http://docs.spring.io/spring/docs/{spring-version}/spring-framework-reference/html/jdbc.html#jdbc-JdbcTemplate
+:sm-statecontext: http://docs.spring.io/spring-statemachine/docs/{spring-statemachine-version}/api/org/springframework/statemachine/StateContext.html
 
 = Spring Statemachine - Reference Documentation
 

docs/src/reference/asciidoc/sm.adoc

@@ -633,7 +633,7 @@ include::samples/DocsConfigurationSampleTests.java[tags=snippet6]
 
 [[sm-statecontext]]
 == Using StateContext
-_StateContext_ is a domain object representing a current status of a
+{sm-statecontext}[_StateContext_] is a domain object representing a current status of a
 state machine within a transition or an action. Context gives an
 access to a various information like event, message headers, extended
 state variables, current transition and a top-level state machine in