Allow exclusions of transitive dependencies

Addresses #1727 
For dependencies that *belong* in the core project, but cause problems with some of the sub modules, we can use exclusion rules so that the sub modules ignore these dependencies.


Generating a project with gdx-tools will add exclusion rules to sub modules that are incompatible, see generated build script below:

```groovy
buildscript {
    repositories {
        mavenCentral()
        mavenLocal()
        maven { url 'https://github.com/steffenschaefer/gwt-gradle-plugin/raw/maven-repo/' }
    }
    dependencies {
        classpath 'de.richsource.gradle.plugins:gwt-gradle-plugin:0.3'
        classpath 'com.android.tools.build:gradle:0.9+'
        classpath 'com.github.jtakakura:gradle-robovm-plugin:0.0.7'
    }
}

allprojects {
    apply plugin: "eclipse"
    apply plugin: "idea"

    version = '1.0'
    ext {
        appName = 'ExclusionTest'
        gdxVersion = '1.0-SNAPSHOT'
        roboVMVersion = '0.0.11'
    }

    repositories {
        mavenLocal()
        mavenCentral()
        maven { url "https://oss.sonatype.org/content/repositories/snapshots/" }
    }
}

project(":core") {
    apply plugin: "java"


    dependencies {
        compile "com.badlogicgames.gdx:gdx:$gdxVersion"
        compile "com.badlogicgames.gdx:gdx-box2d:$gdxVersion"
        compile "com.badlogicgames.gdx:gdx-tools:$gdxVersion"
    }
}

project(":desktop") {
    apply plugin: "java"


    dependencies {
        compile project(":core")
        compile "com.badlogicgames.gdx:gdx-backend-lwjgl:$gdxVersion"
        compile "com.badlogicgames.gdx:gdx-platform:$gdxVersion:natives-desktop"
        compile "com.badlogicgames.gdx:gdx-box2d-platform:$gdxVersion:natives-desktop"
        compile "com.badlogicgames.gdx:gdx-tools:$gdxVersion"
    }
}

project(":android") {
    apply plugin: "android"

    configurations { natives }

    dependencies {
        compile(project(":core")) {
            exclude module: "gdx-tools"
        }
        compile "com.badlogicgames.gdx:gdx-backend-android:$gdxVersion"
        natives "com.badlogicgames.gdx:gdx-platform:$gdxVersion:natives-x86"
        natives "com.badlogicgames.gdx:gdx-platform:$gdxVersion:natives-armeabi"
        natives "com.badlogicgames.gdx:gdx-platform:$gdxVersion:natives-armeabi-v7a"
        natives "com.badlogicgames.gdx:gdx-platform:$gdxVersion:natives-x86"
        compile "com.badlogicgames.gdx:gdx-box2d:$gdxVersion"
        natives "com.badlogicgames.gdx:gdx-box2d-platform:$gdxVersion:natives-armeabi"
        natives "com.badlogicgames.gdx:gdx-box2d-platform:$gdxVersion:natives-armeabi-v7a"
        natives "com.badlogicgames.gdx:gdx-box2d-platform:$gdxVersion:natives-x86"
    }
}

project(":ios") {
    apply plugin: "java"
    apply plugin: "robovm"

    configurations { natives }

    dependencies {
        compile(project(":core")) {
            exclude module: "gdx-tools"
        }
        compile "org.robovm:robovm-rt:${roboVMVersion}"
        compile "org.robovm:robovm-cocoatouch:${roboVMVersion}"
        compile "com.badlogicgames.gdx:gdx-backend-robovm:$gdxVersion"
        natives "com.badlogicgames.gdx:gdx-platform:$gdxVersion:natives-ios"
        natives "com.badlogicgames.gdx:gdx-box2d-platform:$gdxVersion:natives-ios"
    }
}

project(":html") {
    apply plugin: "gwt"
    apply plugin: "war"


    dependencies {
        compile(project(":core")) {
            exclude module: "gdx-tools"
        }
        compile "com.badlogicgames.gdx:gdx-backend-gwt:$gdxVersion"
        compile "com.badlogicgames.gdx:gdx:$gdxVersion:sources"
        compile "com.badlogicgames.gdx:gdx-backend-gwt:$gdxVersion:sources"
        compile "com.badlogicgames.gdx:gdx-box2d:$gdxVersion:sources"
        compile "com.badlogicgames.gdx:gdx-box2d-gwt:$gdxVersion:sources"
    }
}

tasks.eclipse.doLast {
    delete ".project"
}
```