Add processor to generate configuration meta-data

Adds an annotation processor to generates a JSON meta-data file at
compile time from @ConfigurationProperties items. Each meta-data file
can include an array or 'properties' and 'groups'.

A 'property' is a single item that may appear in a Spring Boot
'application.properties' file with a given value. For example,
'server.port' and 'server.context-path' are properties. Each property
may optionally include 'type' and 'description' attributes to provide
the data type (e.g. `java.lang.Integer`, `java.lang.String`) and
some short documentation (taken from the field javadoc) about what the
property is for. For consistency, the type of a primitive is translated
to its wrapper counterpart, i.e. `boolean` becomes `java.lang.Boolean`.

A 'group' provides a higher level grouping of properties. For example
the 'server.port' and 'server.context-path' properties are in the
'server' group.

Both 'property' and 'group' items may additional have 'sourceType' and
'sourceMethod' attributes to indicate the source that contributed them.

Users may use `META-INF/additional-spring-configuration-metadata.json`
to manually provide additionally meta-data that is not covered by
@ConfigurationProperties objects. The contents of this file will be
read and merged with harvested items. The complete meta-data file is
finally written to `META-INF/spring-configuration-metadata.json`.

See gh-1001
Add a defaultValue attribute to config meta-data

Update `ConfigurationMetadataAnnotationProcessor` to include the
'defaultValue' of a property when possible. For example the
'defaultValue' or 'server.port' is '8080'.

Default values are detected by inspecting the field assignments of
@ConfigurationProperties items. In order to detect field values some
internals of the Java compiler are used. To save a dependency on
'tools.jar' internal javac classes are accessed using reflection.

See gh-1001
Generate standard configuration meta-data

Update the `spring-boot`, `spring-boot-autoconfigure` and
`spring-boot-actuator` project to generate configuration meta-data
files during compilation.

See gh-1001
Add @NestedConfigurationProperty meta-data support

Add a @NestedConfigurationProperty annotation which can be used to
customize how configuration mete-data is generated.

Prior to this commit only inner-classes where considered nested
(see Tomcat in ServerProperties). Using this new annotation, the Ssl
property in ServerProperties can be detected as well.

See gh-1001
Document configuration meta-data

Add an appendix to the reference documentation describing the format
of configuration meta-data and how the annotation processor can be used.

Closes gh-1001
Merge branch 'gh-1001'