added "delete passphrase" method to keychain access

main/keychain/src/main/java/org/cryptomator/keychain/KeychainAccess.java

@@ -16,4 +16,11 @@ public interface KeychainAccess {
 	 */
 	CharSequence loadPassphrase(String key);
 
+	/**
+	 * Deletes a passphrase with a given key.
+	 * 
+	 * @param key Unique key previously used while {@link #storePassphrase(String, CharSequence) storing a passphrase}.
+	 */
+	void deletePassphrase(String key);
+
 }

main/keychain/src/main/java/org/cryptomator/keychain/MacSystemKeychainAccess.java

@@ -38,4 +38,9 @@ class MacSystemKeychainAccess implements KeychainAccessStrategy {
 		return SystemUtils.IS_OS_MAC_OSX && keychain != null;
 	}
 
+	@Override
+	public void deletePassphrase(String key) {
+		keychain.deletePassword(key);
+	}
+
 }

main/keychain/src/main/java/org/cryptomator/keychain/WindowsSystemKeychainAccess.java

@@ -37,6 +37,12 @@ class WindowsSystemKeychainAccess implements KeychainAccessStrategy {
 		return null;
 	}
 
+	@Override
+	public void deletePassphrase(String key) {
+		// TODO Auto-generated method stub
+
+	}
+
 	@Override
 	public boolean isSupported() {
 		return SystemUtils.IS_OS_WINDOWS && keyStore != null;

main/keychain/src/test/java/org/cryptomator/keychain/MapKeychainAccess.java

@@ -17,6 +17,12 @@ class MapKeychainAccess implements KeychainAccessStrategy {
 		return map.get(key);
 	}
 
+	@Override
+	public void deletePassphrase(String key) {
+		// TODO Auto-generated method stub
+
+	}
+
 	@Override
 	public boolean isSupported() {
 		return true;