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/context.go b/context.go
index 1b5d390..a946468 100644
--- a/context.go
+++ b/context.go
@@ -77,14 +77,14 @@
 	return Provider{}, ErrProvider
 }
 
-// Instantiates a Cloud Servers object for the provider given.
-func (c *Context) ComputeApi(acc AccessProvider, criteria ApiCriteria) (ComputeProvider, error) {
+// Instantiates a Cloud Servers API for the provider given.
+func (c *Context) ServersApi(acc AccessProvider, criteria ApiCriteria) (CloudServersProvider, error) {
 	url := acc.FirstEndpointUrlByCriteria(criteria)
 	if url == "" {
 		return nil, ErrEndpoint
 	}
 
-	gcp := &genericCloudProvider{
+	gcp := &genericServersProvider{
 		endpoint: url,
 		context:  c,
 		access:   acc,