Introduce java.nio.file.Path based Resource

Develop new org.springframework.core.io.Resource implementation
backed by java.nio.file.Path. Primarily developed to allow custom
file system implementations to be used with Spring.

Since the minimum requirement for Spring is still Java 6 the
existing FileSystemResource can't be retrofitted (and no #getPath
method can be added to the Resource interface).

Unlike FileSystemResource, PathResource delegates to the underlying
file system instead of StringUtils. It has therefore slightly
different semantics. First, when building relative resources via
createRelative the relative path will apply to this path (like URL or
Unix). Second, equality is delegated to the underlying file system
provider so it's case-insensitive on Windows.

Issue: SPR-10608
Merge pull request #295 from marschall/SPR-10608

# By Philippe Marschall
* SPR-10608-rebase:
  Introduce java.nio.file.Path based Resource