This commit introduces initial support for JUnit Jupiter (i.e., the new
programming and extension models in JUnit 5) in the Spring TestContext
Framework.

Specifically, this commit introduces the following.

- SpringExtension: an implementation of multiple extension APIs from
  JUnit Jupiter that provides full support for the existing feature set
  of the Spring TestContext Framework. This support is enabled via
  @ExtendWith(SpringExtension.class).

- @SpringJUnitConfig: a composed annotation that combines
  @ExtendWith(SpringExtension.class) from JUnit Jupiter with
  @ContextConfiguration from the Spring TestContext Framework.

- @SpringJUnitWebConfig: a composed annotation that combines
  @ExtendWith(SpringExtension.class) from JUnit Jupiter with
  @ContextConfiguration and @WebAppConfiguration from the Spring
  TestContext Framework.

Issue: SPR-13575

Issue: SPR-13188

Issue: SPR-14362

Issue: SPR-14420

Issue: SPR-14370

Issue: SPR-14402

Issue: SPR-14424

Issue: SPR-14423

Closes gh-1097

Latest dependency updates (HttpAsyncClient 4.1.2, HtmlUnit 2.22, Selenium 2.53.1, JOpt Simple 5.0.2)

Issue: SPR-14422

Issue: SPR-14327

Closes gh-967

* pr/1089:
  Polish
  Fix typo in asciidoc

Closes gh-1089

See gh-1089

Closes gh-1096

* pr/1095:
  Polish contribution
  Fix typo in javadoc

Closes gh-1095

See gh-1095

Issue: SPR-14420

Issue: SPR-14410

Issue: SPR-14321

Issue: SPR-14415

Originally handleNoMatch looked for partial matches based on URL
pattern, HTTP method, consumes, produces, and params in that order
but without narrowing down the set of partial matches resulting in
potentially inaccruate response status codes

Commit 473de0 added an improvement to narrow the set with partial
matches for URL pattern and HTTP method matches.

This commit overhauls handleNoMatch so that the narrowing down of
matches happens at each stage resulting in more accurate error
reporting for request mappings with fine-grained conditions.

Issue: SPR-14397

This commit fixes MessagingMessageConverter to no longer expose the lazy
message resolution algorithm. This restores proper behaviour for
converters used outside of that context.

Instead, such arrangement is now private to
AbstractAdaptableMessageListener (as it should).

Issue: SPR-14389