Server Metadata Update (#93)

This commit updates the Server Metadata so it can be parsed and
translated using JSON tags.

The Server Metadata result has been changed to map[string]string to
match the request.

A unit test was added to verify the above and a piece of metadata has
been added to the server creation acceptance test.
diff --git a/openstack/compute/v2/servers/requests.go b/openstack/compute/v2/servers/requests.go
index 0210c2a..fcc8b2e 100644
--- a/openstack/compute/v2/servers/requests.go
+++ b/openstack/compute/v2/servers/requests.go
@@ -156,7 +156,7 @@
 	Networks []Network `json:"-"`
 
 	// Metadata contains key-value pairs (up to 255 bytes each) to attach to the server.
-	Metadata map[string]string `json:"-"`
+	Metadata map[string]string `json:"metadata,omitempty"`
 
 	// Personality includes files to inject into the server at launch.
 	// Create will base64-encode file contents for you.