Updated EEPROM storage class

To avoid having a .cpp just for an extern variable definition, `static`
has been chosen over `extern`.

As the `EEPROMClass` class simply wraps functionality located elsewhere,
it is completely compiled away. Even though each translation unit which
includes the header will get a copy with internal linkage, there is no
associated overhead.

More info
[here](http://stackoverflow.com/questions/29098518/extern-variable-only-in-header-unexpectedly-working-why)

libraries/EEPROM/EEPROM.h

@@ -154,5 +154,5 @@ struct EEPROMClass{
     }
 };
 
-extern EEPROMClass EEPROM;
+static EEPROMClass EEPROM;
 #endif