Support enumerating enabled packages -

[Gestas]

We should support enumerating enabled and running Synology packages. This will make the sample code in this repo more tolerant to different customer configurations. I'll include this in the pull for #58.

print("=== Enabled Packages ===")
   ...: api.polling.update()
   ...: for p in api.polling.enabled_packages:
   ...:     print(p)
   ...: print("--")

=== Enabled Packages ===
CloudSync
Docker
LogCenter
OAuthService
SnapshotReplication
StorageAnalyzer
SynoFinder
VPNCenter
exFAT
--