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/global_context.go b/global_context.go
index b071c09..c89ac17 100644
--- a/global_context.go
+++ b/global_context.go
@@ -43,6 +43,6 @@
 }
 
 // Instantiates a Cloud Servers object for the provider given.
-func ComputeApi(acc AccessProvider, criteria ApiCriteria) (ComputeProvider, error) {
-	return globalContext.ComputeApi(acc, criteria)
+func ServersApi(acc AccessProvider, criteria ApiCriteria) (CloudServersProvider, error) {
+	return globalContext.ServersApi(acc, criteria)
 }