Support @Grab when app has multiple groovy scripts

The AST transformation that processes @Grab annotations is driven once
per source file. Previously, this meant that if an app consisted of
multiple source files then multiple, discrete dependency resolutions
would be performed.

This commit updates AetherGrapeEngine to cache a previous resolution's
outcome and use its dependency to influence the outcome of subsequent
resolutions. For example if a one resolution results in spring-core
4.0.0.RELEASE being added to the classpath, subsequent resolutions
that depend upon spring-core will always get the 4.0.0.RELEASE
version. This is achieved by using the dependencies found by earlier
resolutions as dependency management configuration of the current
resolution. This removes the possibility of multiple versions of the
same dependency ending up on the classpath.

In addition to using the results of earlier resolutions to provide
dependency management configuration, default dependency management
configuration is also provided. This configuration is specified by
the springcli.properties file and ensures that, where Boot prescribes
certain versions of a dependency, that is the version that will be
resolved. For example, this ensures that spring-data-redis, which
depends upon Spring 3.1.4, pulls in the version of Spring that Boot
requires instead.

Fixes #224