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/acceptance/02-list-servers.go b/acceptance/02-list-servers.go
index b33a15c..bd1da3c 100644
--- a/acceptance/02-list-servers.go
+++ b/acceptance/02-list-servers.go
@@ -30,7 +30,7 @@
 		panic(err)
 	}
 
-	api, err := gophercloud.ComputeApi(acc, gophercloud.ApiCriteria{
+	api, err := gophercloud.ServersApi(acc, gophercloud.ApiCriteria{
 		Name:      "cloudServersOpenStack",
 		Region:    "DFW",
 		VersionId: "2",