xdb: implement public API

[tsatke]

Implement Go API to directly interact with a database.

Should be usable similar to the following.

db, err := xdb.Open(myAferoDbFile)
db.CreateTable(xdb.Table{
    Name: "myTable",
    Columns: []xdb.CreateTableColumn{
        {Name: "col1", Type: xdb.TypeInteger},
        {Name: "col2", Type: xdb.TypeString},
    },
})

• open / create ( xdb: create / open #39 )
• create table ( xdb: create table #38 )
• select ( xdb: select #40 )
• insert ( xdb: insert #41 )
• delete ( xdb: delete #42 )
• drop ( xdb: drop #43 )