Added concept of platform to the internals.

src/test/groovy/net/rubygrapefruit/platform/PosixFileTest.groovy

@@ -6,7 +6,7 @@ import org.junit.rules.TemporaryFolder
 
 class PosixFileTest extends Specification {
     @Rule TemporaryFolder tmpDir
-    final PosixFile file = Platform.get(PosixFile.class)
+    final PosixFile file = Native.get(PosixFile.class)
 
     def "can set mode on a file"() {
         def testFile = tmpDir.newFile("test.txt")

src/test/groovy/net/rubygrapefruit/platform/ProcessTest.groovy

@@ -6,7 +6,7 @@ import spock.lang.Specification
 
 class ProcessTest extends Specification {
     @Rule TemporaryFolder tmpDir
-    final Process process = Platform.get(Process.class)
+    final Process process = Native.get(Process.class)
 
     def "can get PID"() {
         expect:

src/test/groovy/net/rubygrapefruit/platform/TerminalTest.groovy

@@ -6,7 +6,7 @@ import spock.lang.Specification
 
 class TerminalTest extends Specification {
     @Rule TemporaryFolder tmpDir
-    final TerminalAccess terminal = Platform.get(TerminalAccess.class)
+    final TerminalAccess terminal = Native.get(TerminalAccess.class)
 
     def "can check if attached to terminal"() {
         expect: