Use system line separator in MultipleFailuresError

Use of "\n" as the line separator in MultipleFailuresError caused
tests to fail on Windows.

This commit fixes this by using the system line separator.

Issue: #29

src/main/java/org/opentest4j/MultipleFailuresError.java

@@ -54,17 +54,20 @@ public String getMessage() {
 			return this.heading;
 		}
 
+		String lineSeparator = System.getProperty("line.separator");
+
 		// @formatter:off
 		StringBuilder builder = new StringBuilder(this.heading)
 				.append(" (")
 				.append(failureCount).append(" ")
 				.append(pluralize(failureCount, "failure", "failures"))
-				.append(")\n");
+				.append(")")
+				.append(lineSeparator);
 		// @formatter:on
 
 		int lastIndex = failureCount - 1;
 		for (AssertionError failure : this.failures.subList(0, lastIndex)) {
-			builder.append("\t").append(nullSafeMessage(failure)).append("\n");
+			builder.append("\t").append(nullSafeMessage(failure)).append(lineSeparator);
 		}
 		builder.append('\t').append(nullSafeMessage(this.failures.get(lastIndex)));