Add conditional-close example.

Since the API documentation might be a little unclear:

https://github.com/veox/krakenex/issues/1

examples/conditional-close.py

@@ -0,0 +1,23 @@
+#!/usr/bin/env python2
+#
+# Demonstrates how to use the conditional close functionality; that is,
+# placing an order that, once filled, will place another order.
+#
+# This can be useful for very simple automation, where a bot is not
+# needed to constantly monitor execution.
+
+import krakenex
+
+k = krakenex.API()
+k.load_key('kraken.key')
+
+k.query_private('AddOrder', {'pair': 'XXBTZEUR',
+                             'type': 'buy',
+                             'ordertype': 'limit',
+                             'price': '1',
+                             'volume': '1',
+                             'close[pair]': 'XXBTZEUR',
+                             'close[type]': 'sell',
+                             'close[ordertype]': 'limit',
+                             'close[price]': '9001',
+                             'close[volume]': '1'})

examples/kraken.key

@@ -0,0 +1,2 @@
+keykeykeykeykeykeykeykeykeykeykeykeykeykeykeykeykeykeykey
+secretsecretsecretsecretsecretsecretsecretsecretsecretsecretsecretsecretsecretsecretsecret