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)
