Ability to specify Scala installation directory

These patches introduce scalaHome configuration parameter and scala.home property that allow specifying Scala installation directory. If either of them is used, then maven-scala-plugin will use Scala JARs from the specified directory rather than from Maven artifacts.

With these patches, the following pom can be used to quickly switch between different Scala installations:

    <project xmlns="http://maven.apache.org/POM/4.0.0"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
      <modelVersion>4.0.0</modelVersion>
      <groupId>org</groupId>
      <artifactId>example</artifactId>
      <packaging>jar</packaging>
      <version>1.0-SNAPSHOT</version>
      <name>example</name>
      <profiles>
        <profile>
          <id>scala-home</id>
          <activation>
            <property>
              <name>scala.home</name>
            </property>
          </activation>
          <dependencies>
            <dependency>
              <groupId>org.scala-lang</groupId>
              <artifactId>scala-library</artifactId>
              <version>2.9.0-SNAPSHOT</version>
              <scope>system</scope>
              <systemPath>${scala.home}/lib/scala-library.jar</systemPath>
            </dependency>
          </dependencies>
        </profile>
        <profile>
          <id>scala-artifact</id>
          <activation>
            <property>
              <name>!scala.home</name>
            </property>
          </activation>
          <dependencies>
            <dependency>
              <groupId>org.scala-lang</groupId>
              <artifactId>scala-library</artifactId>
              <version>2.9.0-SNAPSHOT</version>
            </dependency>
          </dependencies>
        </profile>
      </profiles>
      <build>
        <plugins>
          <plugin>
            <groupId>org.scala-tools</groupId>
            <artifactId>maven-scala-plugin</artifactId>
            <version>2.15.3-SNAPSHOT</version>
            <configuration>
              <scalaVersion>2.9.0-SNAPSHOT</scalaVersion>
            </configuration>
          </plugin>
        </plugins>
      </build>
    </project>