Rename ComputeApi to ServerApi

ComputeApi never sat well with me.  OpenStack documents prefer to call
their computing service "Cloud Server" API instead.  So, I renamed all
public-facing types and procedure names to reflect this usage.

Fixes #33
diff --git a/servers_test.go b/servers_test.go
index e296801..46ba6db 100644
--- a/servers_test.go
+++ b/servers_test.go
@@ -28,7 +28,7 @@
 		internal: "http://localhost:8086",
 	}
 
-	_, err := c.ComputeApi(acc, ApiCriteria{
+	_, err := c.ServersApi(acc, ApiCriteria{
 		Name:      "cloudComputeOpenStack",
 		Region:    "dfw",
 		VersionId: "2",