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/interfaces.go b/interfaces.go
index 63439c5..de2c35b 100644
--- a/interfaces.go
+++ b/interfaces.go
@@ -15,8 +15,8 @@
 	AuthToken() string
 }
 
-// ComputeProvider instances encapsulate a Cloud Servers API, should one exist in the service catalog
+// CloudServersProvider instances encapsulate a Cloud Servers API, should one exist in the service catalog
 // for your provider.
-type ComputeProvider interface {
+type CloudServersProvider interface {
 	ListServers() ([]Server, error)
 }