feat: add read-only support of the meta API (#13)

* Add bases API.

* Implement get tables schema

* Rename GetBase to GetBaseSchema

* Add tests

* Update doc

base_test.go

@@ -0,0 +1,25 @@
+package airtable
+
+import (
+	"testing"
+)
+
+func TestGetBaseSchema(t *testing.T) {
+	client := testClient(t)
+	baseschema := client.GetBaseSchema("test")
+	baseschema.client.baseURL = mockResponse("base_schema.json").URL
+
+	result, err := baseschema.Do()
+	if err != nil {
+		t.Errorf("there should not be an err, but was: %v", err)
+	}
+	if len(result.Tables) != 2 {
+		t.Errorf("there should be 2 tales, but was %v", len(result.Tables))
+	}
+
+	baseschema.client.baseURL = mockErrorResponse(400).URL
+	_, err = baseschema.Do()
+	if err == nil {
+		t.Errorf("there should be an err, but was nil")
+	}
+}