Log API diffs using ShowAPI and java-diff-utils library.

Implement displaying API changes by using textual representation
of an API (ShowAPI) and good, old textual diff algorithm. We are
using java-diff-utils library that is distributed under Apache 2.0
license.

Notice that we have only soft dependency on java-diff-utils. It means
that we'll try to lookup java-diff-utils class through reflection
and fail gracefully if none is found on the classpath. This way
sbt is not getting any new dependency. If user needs to debug
api diffs then it's matter of starting sbt with
`-Dsbt.extraClasspath=path/to/diffutils.jar` option passed
to sbt launcher.
Make api diff context size valuable configurable.

Add `apiDiffContextSize` option to `IncOptions` which allows one
to control the size (in lines) of a context used when printing
diffs for textual API representation.

The default value for `apiDiffContextSize` is 5 which seems to be
enough for most situations. This is verified by many debugging
sessions I performed when using api diffing functionality.