Add support for tsql 3 and 4 part naming

[aoswalt]

Tables may be referenced in SQL Server by up to 4-part naming of
[server].[database].[schema].[table].

These may be passed to the Tds adapter via the schema_prefix as tuples:
@schema_prefix {"database", "schema"} and
@schema_prefix {"server", "database", "schema"}

Fixes #278

[josevalim]

All of these map joins are returning the structure itself, i don't think you need them:

Suggested change

	defp quote_table({server, db, schema}, name),
	do: Enum.map_join([quote_table(server), ".", quote_table(db), ".", quote_table(schema), ".", quote_table(name)], "", &"#{&1}")
	defp quote_table({db, schema}, name),
	do: Enum.map_join([quote_table(db), ".", quote_table(schema), ".", quote_table(name)], "", &"#{&1}")
	defp quote_table(prefix, name),
	do: Enum.map_join([quote_table(prefix), ".", quote_table(name)], "", &"#{&1}")
	defp quote_table({server, db, schema}, name),
	do: [quote_table(server), ".", quote_table(db), ".", quote_table(schema), ".", quote_table(name)]
	defp quote_table({db, schema}, name),
	do: [quote_table(db), ".", quote_table(schema), ".", quote_table(name)]
	defp quote_table(prefix, name),
	do: [quote_table(prefix), ".", quote_table(name)]

Also, please add tests to test/ecto/adapters/tds_test.exs. See "delete_all with prefix" as an example, except you don't need to call delete_all, all is fine. Thanks!

[michalmuskala]

Should it use a list of strings instead of a tuple if there can be many elements? This seems more natural to me

[josevalim]

I thoughT the same but given the number of elements is still bound (3), I think a tuple works better.