Provides instructions for what to do in the event of an unmapped dependency

Closes #34
Update naming to match Gradle conventions

Closes #33
Remove usage of shaded Ant classes

The shaded Ant classes are not always available. Previously, 
org.apache.maven.artifact.ant.shaded.Os was used to determine if the
plugin is running on Windows. This commit removes this usage in favour
of checking the value of File.pathSeparatorChar (it's ';' on Windows and
':' on Unix-like platforms).
Consume versions from one or more external dependencies

Previously, the Spring IO versions were embedded in the plugin, thereby
tightly coupling the plugin to a particular version of Spring IO. This
commit updates the plugin to consume versions from one or more external
dependencies that have been added to the springioVersions configuration.
For example:

dependencies {
	springIoVersions 'io.spring.platform:platform-versions:1.0.0.BUILD-SNAPSHOT@properties'
}

Closes #31, #32