Allow an empty string as a default in variable substitution.

Lanucher configuration parser would previously require a default
value in variable substitution pattern to be non-empty string.
This is an unnecessary restriction as empty value is sometimes
useful as in this example:

  resources: ${sbt.extraClasspath-}

This commit lifts this restriction so empty default values are
allowed. The change has been discussed with @harrah.
Configure additional classpath through `sbt.extraClasspath`

Modify the default launcher configuration for sbt so extra classpath
entries for sbt can be configured through `sbt.extraClasspath` system
property.
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.
Merge api-diff into 0.13
make APIDiff private[inc]