DATACASS-512 - Support exists and count projections.

We now support exists and count projections derived from query methods and on Template API level.

Exists queries fetch rows limiting to a single row to optimize fetching.

Exists queries also support count results, i.e. a single row with a single numeric column is considered a count result. It's evaluated by comparing the number being greater to zero.

boolean exists = template.exists(Query.query(where("firstname").is("Walter")), Person.class);

long count = template.count(Query.query(where("lastname").is("White")), Person.class);

interface PersonRepository extends Repository<Person, String> {

    long countByLastname(String lastname); // derived query

    boolean existsByLastname(String lastname);  // derived query

    @CountQuery("SELECT COUNT(*) from users WHERE lastname = ?0")
    long countQueryByLastname(String lastname); // String-based query

    @ExistsQuery("SELECT * from users WHERE lastname = ?0 LIMIT 1")
    boolean existsQueryByLastname(String lastname);  // String-based query
}

Remove superfluous throws declarations.

src/main/asciidoc/new-features.adoc

@@ -3,6 +3,11 @@
 
 This chapter summarizes changes and new features for each release.
 
+[[new-features.2-1-0]]
+== What's new in Spring Data for Apache Cassandra 2.1
+* New annotations for `@CountQuery` and `@ExistsQuery`.
+* Template API extended with `count(…)` and `exists(…)` methods accepting `Query`.
+
 [[new-features.2-0-0]]
 == What's new in Spring Data for Apache Cassandra 2.0