Add API for table memo and schema memo

[ragi256]

Add API functionality to Dmemo to make it easier to combine DWH environments.
If Dmemo can be treated not only as a document site but also as an API server, it will be more useful.

For example

• auto complete of table columns for SQL Editor (such as bdash)
• auto link to Dmemo in SQL share web app ( such as bdash server)
• cache layer for meta data of DWH DB