Assert context uniqueness against merged config

Prior to this commit, the uniqueness check for @ContextConfiguration
attributes within a @ContextHierarchy was performed at a single test
class level instead of against the merged configuration for all test
class levels in the test class hierarchy.

This commit addresses this issue by moving the uniqueness check
algorithm from resolveContextHierarchyAttributes() to
buildContextHierarchyMap() within ContextLoaderUtils.

Issue: SPR-10997
Add targetIsClass to SpEL property cache key

Update the `CacheKey` class used by `ReflectivePropertyAccessor` to
include if the target object is class. The prevents an incorrect cache
hit from being returned when a property with the same name is read on
both an object and its class. For example:

	#{class.name}
	#{name}

Issue: SPR-10486
Multicaster support for events with null source

Update `AbstractApplicationEventMulticaster` to support `EventObjects`
with a null source (which can happen if they have been serialized).

Issue: SPR-10945
Consistent ordering for @PropertySource locations

Ensure that property source locations are processed in the same order
regardless if the 'name' attribute is set or not.

Prior to this commit multiple locations from a `@PropertySource` with
a name were added to a `CompositePropertySource` in such a way that
the first location would take precedence. This has now been reversed
for consistence with unnamed `@PropertySource`s

Issue: SPR-10820
Polish (minor)
Introduced MvcUriComponentsBuilder to create URIs pointing to controller methods.

MvcUriComponentsBuilder allows creating URIs that point to Spring MVC
controller methods annotated with @RequestMapping. It builds them by
exposing a mock method invocation API similar to Mockito, records the
method invocations and thus builds up the URI by inspecting the mapping
annotations and the parameters handed into the method invocations.

Introduced a new SPI UriComponentsContributor that should be implemented 
by HandlerMethodArgumentResolvers that actually contribute path segments 
or query parameters to a URI. While the newly introduced 
MvcUriComponentsBuilder looks up those UriComponentsContributor instances 
from the MVC configuration.

The MvcUriComponentsBuilderFactory (name to be discussed - MvcUris maybe?) 
prevents the multiple lookups by keeping the UriComponentsBuilder 
instances in an instance variable. So an instance of the factory could 
be exposed as Spring bean or through a HandlerMethodArgumentResolver to 
be injected into Controller methods.

Issue: SPR-10665, SPR-8826
Integrate suggested support for creating MVC URLs

The key contract is MvcUrls. An instance is automatically created with
the Spring MVC namespace and the MVC Java config but can also be easily
created in any configuration.

Some example tests can be found in DefaultMvcUrlsTests.

Issue: SPR-10665, SPR-8826
Polish GenericMessage
Add @PropertySources and ignoreResourceNotFound

Support repeatable @PropertySource annotations in Java 8 and add
@PropertySources container annotation for Java 6/7. Also add an
ignoreResourceNotFound attribute to @PropertySource allowing missing
property resources to be silently ignored.

This commit also introduces some generally useful methods to
AnnotationUtils for working with @Repeatable annotations.

Issue: SPR-8371