Add eachItem method in order to be able to validate each emission

[ppgsalomao]

Description
As a i want to be able to validate each emission from my Observable in a clean way, such as the other methods of this library.

Acceptance Criteria

• Add a eachItem() method that receives a custom matcher and validates each object emitted by the Observable against that matcher.

Example

@Test 
public void multipleEmisstion_AreMatchedOnCondition() {
   List<String> expected = Arrays.asList("These", "are", "Expected", "Values");
   Observable<String> values = Observable.from(expected);

   Condition<String> notNullAndNotEmpty = 
      new Condition<String>() {
         @Override 
         public boolean matches(String value) {
            return value != null && !value.isEmpty();
         }
      };

   RxAssertions.assertThat(values)
         .completes()
         .eachItemMatches(notNullAndNotEmpty);
}