Bring most commonly used classes to default namespace

Author: facchinm

Diff for: api/IPAddress.h

@@ -83,3 +83,5 @@ class IPAddress : public Printable {
 
 extern const IPAddress INADDR_NONE;
 }
+
+using arduino::IPAddress;

Diff for: api/Stream.h

@@ -128,4 +128,6 @@ class Stream : public Print
 
 #undef NO_IGNORE_CHAR
 
-}
+}
+
+using arduino::Stream;

Diff for: api/Udp.h

@@ -87,4 +87,6 @@ class UDP : public Stream {
   uint8_t* rawIPAddress(IPAddress& addr) { return addr.raw_address(); };
 };
 
-}
+}
+
+using arduino::UDP;