Add API class

- Secure token based on username/password

lib/api.dart

@@ -0,0 +1,72 @@
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:path/path.dart' as path;
+import 'package:http/http.dart' as http;
+
+
+class InventreeAPI {
+
+  InventreeAPI(this._username, this._password) {
+    _secureToken();
+  }
+
+  // Base URL for InvenTree API
+  String _base_url = "http://10.0.0.7:8000/api/";
+
+  String _username = "";
+  String _password = "";
+
+  // Authentication token (initially empty, must be requested)
+  String _token = "";
+
+  // Construct an API URL
+  String _makeUrl(String url) {
+    return path.join(_base_url, url);
+  }
+
+  bool _hasToken() {
+    return _token.isNotEmpty;
+  }
+
+  // Request an API token from the server.
+  // A valid username/password combination must be provided
+  void _secureToken() async {
+    _token = "";
+
+    var _url = _makeUrl("user/token/");
+    final response = await http.post(_url,
+        body: {
+          "username": _username,
+          "password": _password
+      });
+
+    if (response.statusCode != 200) {
+      print("Invalid status code:" + String.fromCharCode(response.statusCode));
+    } else {
+      var _json = json.decode(response.body);
+
+      if (_json["token"] != null) {
+        _token = _json["token"];
+        print("Received token: " + _token);
+      }
+    }
+  }
+
+  Future<http.Response> get(String url) async {
+    var _url = _makeUrl(url);
+    final response = await http.get(_url,
+      headers: {
+          HttpHeaders.authorizationHeader: "Token: " + _token
+        }
+    );
+
+    print("Making request to " + _url);
+    print(response.statusCode);
+    print(response.body);
+
+    return response;
+  }
+
+}