Polish Environment Javadoc
Avoid Groovy compilation error

Prior to this change there were multiple Groovy classes named
'GroovyMessenger', causing a compilation error in certain IDE
arrangements.

This change renames one of these classes to GroovyMessenger2, thus
avoiding the problem entirely.
Avoid "cannot extend final class" error

Prior to this change, IDEA failed to run all tests due to confusion
between the two variants of SingletonBeanFactoryLocatorTests across
spring-context and spring-beans. Removing the 'final' modifier from the
latter effectively solves the problem.
Polish build.gradle formatting
Add various IDEA artifacts to .gitignore

 - ignore derby.log wherever it is written
 - ignore IDEA's test-output directory
 - ignore IDEA's Atlassian connector config file
Update import-into-idea.md
Update source and target JDK compatibility

"main" source and target compatibility has been upgraded across the
board from 1.5 to 1.6, meaning that Spring Framework's minimum supported
JDK is 6. Use of @Override annotations on implementations of interface
methods is now permitted.

"test" source and target compatibility has been upgraded to 1.7 except
where noted, allowing us to use 1.7 language features such as
diamond-style (<>) generics declarations, automatic resource management
and multi-catch. More importantly, we will be able to upgrade to 1.8
once it is available in order to make use of lambda expressions, etc in
our test cases.

IDE configurations must be relaxed to allow 1.7 across the board, as
neither Eclipse nor IDEA are clever enough to allow for different
language levels across production and test resources. See [1] for a
feature request on that front.

spring-oxm is a special case here, and has been pinned at 1.6
compatibility even for its test sources in order to avoid a class
verification error that JibX throws when encountering 1.7-level
bytecode [2].

Likewise with spring-orm, toplink encounters a similar class
verification error, so has been pinned to 1.6 for the time being.
When we remove the (already deprecated since 3.2) Toplink support
we can restore compatibility to 1.7.

[1]: http://youtrack.jetbrains.com/issue/IDEA-97814
[2]: http://jira.codehaus.org/browse/JIBX-465
Add @Override annotations where appropriate
Update Xlint compiler warning options

 - Note that we are intentionally ignoring [fallthrough] compiler
   warnings as falling through in case statements is often by design.

 - [options] warnings are also suppressed as we do not have the option
   of referencing an rt.jar other than the one we are compiling and
   building against.
Replace <code> tags in Javadoc with {@code ...}
Remove unused imports
Remove unnecessary @SuppressWarnings annotations (wip)
Fix [dep-ann] warnings with @Deprecated
Fix remaining [serial] compiler warnings

In certain cases and for unknown reasons, a correctly-placed
@SuppressWarnings("serial") annotation would fix the problem at the
javac level (i.e. the Gradle command-line), but would produce an
"unnecessary @SuppressWarnings" warning within Eclipse. In these
cases, a private static final serialVersionUID field has been added
with the default value of 1L.
Fix [rawtypes] compiler warnings (wip)

 - Suppress rawtypes warnings for src/main
 - Enable rawtypes warnings for src/test

Try to resolve all rawtypes problems across tests as aggressively as
possible, i.e. without regard for binary compatibility

So far unable to find a way to bulk-fix these in either Eclipse or IDEA.
Might need to be a manual process after all.
Fix various compiler warnings in spring-context

All warning types other than [deprecation] and [unchecked] are now
fixed in spring-context/src/test