Docs: fix title of 0.13.0 change summary page
API documentation for Project transforming methods
Project.in(File) for setting the base directory of a Project
when looking for the enclosing val definition in definingValName, allow the macro to be in an expression
Support 'val p = project' via a 'project' macro
Convenience method Project.configure(f,g) for g(f(proj)) to be used to apply plugin configuration provided as a Project=>Project transformation
Provide cacheDirectory via Streams instead of a separate setting

This is cleaner and makes it harder for the task author to have overlapping caches.
Disable Ivy debug-level logging (see elaboration below).  Fixes #635.

 The verbose-level logging is what usually contains relevant information for the user.
Persisting the debug logging slows down update noticeably and clutters the more useful
debug logging output from verbose.
Make exclude-classifiers per-user instead of per-build. Fixes #634.
Option to automatically manage API documentation mappings

Set autoAPIMappings := true to enable.

Then, set apiURL to the base URL of the API documentation for a project.
This will get stored in an extra attribute in the ivy.xml or as a property
a pom.xml.

When using managed dependencies that have set their apiURL, the -doc-external-doc
setting for scaladoc will be automatically configured.  Note that this option
will only be available in Scala 2.10.1 and so enabling autoAPIMappings for
earlier versions will result in an error from scaladoc.

For unmanaged dependencies or dependencies without an automatic apiURL, add the
(File,URL) mapping to apiMappings.  The File is the classpath entry and the URL
is the location of the API documentation.
API documentation for ModuleID methods
Split IvyInterface into Artifact, Configuration, ModuleID, and Resolver source files
Docs: add autoAPIMappings, streams.cacheDirectory, and project macro.