Refactoring, better benchmarks.

Extracted interface BulkOperations for custom repository behavior.

Using JUnitBenchmarks for benchmarking.

JdbcTemplate and JDBI implementations are first-class citizens, which
leads to a harmonized test hierarchy.
Updated README.md.
Merge branch 'master' of https://github.com/shimikano/spring-jpa-bulk

Conflicts:
	README.md