Rename API and reference doc output dirs

Prior to this change, aggregated javadoc has been written to an 'api'
dir and reference docs into a 'reference' dir. This shorter naming is
desirable, but is not compatible with historical naming under 3.1.x as
defined by the Ant+Ivy Spring Build.

For example, URLs are currently as follows:

http://static.springsource.org/spring-framework/docs/3.1.3.RELEASE/javadoc-api
http://static.springsource.org/spring-framework/docs/3.1.x/javadoc-api
http://static.springsource.org/spring-framework/docs/current/javadoc-api

Particularly with regard to the 'current' link, we do not want to
break this url scheme with the shortened 'api' directory naming. Of
course this compatibility can be preserved with symlinks, but this makes
the release process that much more complicated and fragile.

This commit ensures that the naming is 'javadoc-api' and
'spring-framework-reference', consistent with historical convention.
We can always add a symlinking scheme to add the shorter 'api' and
'reference' options after the fact if desired.
Eliminate all Javadoc warnings

 - Support external Javadoc links using Gradle's javadoc.options.links

 - Fix all other Javadoc warnings, such as typos, references to
   non-existent (or no longer existent) types and members, etc,
   including changes related to the Quartz 2.0 upgrade (SPR-8275) and
   adding the HTTP PATCH method (SPR-7985).

 - Suppress all output for project-level `javadoc` tasks in order to
   hide false-negative warnings about cross-module @see and @link
   references (e.g. spring-core having a @see reference to spring-web).
   Use the `--info` (-i) flag to gradle at any time to see project-level
   javadoc warnings without running the entire `api` task. e.g.
   `gradle :spring-core:javadoc -i`

 - Favor root project level `api` task for detection of legitimate
   Javadoc warnings. There are now zero Javadoc warnings across the
   entirety of spring-framework. Goal: keep it that way.

 - Remove all @link and @see references to types and members that exist
   only in Servlet <= 2.5 and Hibernate <= 4.0, favoring 3.0+ and 4.0+
   respectively. This is necessary because only one version of each of
   these dependencies can be present on the global `api` javadoc task's
   classpath. To that end, the `api` task classpath has now been
   customized to ensure that the Servlet 3 API and Hibernate Core 4 jars
   have precedence.

 - SPR-8896 replaced our dependency on aspectjrt with a dependency on
   aspectjweaver, which is fine from a POM point of view, but causes
   a spurious warning to be emitted from the ant iajc task that it
   "cannot find aspectjrt on the classpath" - even though aspectjweaver
   is perfectly sufficient. In the name of keeping the console quiet, a
   new `rt` configuration has been added, and aspectjrt added as a
   dependency to it. In turn, configurations.rt.asPath is appended to
   the iajc classpath during both compileJava and compileTestJava for
   spring-aspects.

Issue: SPR-10078, SPR-8275, SPR-7985, SPR-8896
Upgrade to AspectJ 1.7.1

This change upgrades the spring-framework build to AspectJ 1.7.1 to
ensure compatibility. We remain backward-compatible to at least AJ
1.6.12 (the previous version), and likely back to 1.6.7.

AJ 1.7 allows for weaving Java 7 bytecode, particularly with regard
to invokedynamic instructions, and furthermore makes improvements to
the aspectj language itself to allow users to take advantage of Java 7-
style language features within aspects. See [1] for details.

[1]: http://eclipse.org/aspectj/doc/released/README-170.html

Issue: SPR-10079