Fix duration parse issue.

Duration strings that consist of a single "-" or "+" character were
throwing an IndexOutOfBoundsException. An IllegalArgumentException
should be thrown instead because it's not a valid duration string.

#127

src/main/java/biweekly/util/Duration.java

@@ -89,7 +89,7 @@ public static Duration parse(String value) {
 			index++;
 		}
 
-		if (value.charAt(index) != 'P') {
+		if (index >= value.length() || value.charAt(index) != 'P') {
 			throw parseError(value);
 		}
 

src/test/java/biweekly/util/DurationTest.java

@@ -98,6 +98,16 @@ public void parse_unknown_character_after_9() {
 		Duration.parse("P5Z");
 	}
 
+	@Test(expected = IllegalArgumentException.class)
+	public void parse_single_hyphen() {
+		Duration.parse("-");
+	}
+
+	@Test(expected = IllegalArgumentException.class)
+	public void parse_single_plus() {
+		Duration.parse("+");
+	}
+
 	@Test(expected = IllegalArgumentException.class)
 	public void parse_empty_string() {
 		Duration.parse("");