DATAMONGO-1492 - Make o.s.d.m.core.aggregation.AggregationExpression public.

By turning `AggregationExpression` public we allow adding custom expressions without workarounds. It is now possible to create eg. `ProjectionOperation` like:

ProjectionOperation agg = Aggregation.project()
      .and(new AggregationExpression() {

        @Override
        public DBObject toDbObject(AggregationOperationContext context) {

          DBObject filterExpression = new BasicDBObject();
          filterExpression.put("input", "$x");
          filterExpression.put("as", "y");
          filterExpression.put("cond", new BasicDBObject("$eq", Arrays.<Object> asList("$$y.z", 2)));

          return new BasicDBObject("$filter", filterExpression);
        }
      }).as("profile");

Original pull request: #392.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AggregationExpression.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2015 the original author or authors.
+ * Copyright 2015-2016 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -23,8 +23,9 @@ import com.mongodb.DBObject;
  *
  * @author Thomas Darimont
  * @author Oliver Gierke
+ * @author Christoph Strobl
  */
-interface AggregationExpression {
+public interface AggregationExpression {
 
 	/**
 	 * Turns the {@link AggregationExpression} into a {@link DBObject} within the given