Fix error with uninitialized index.
Get rid of macros.
Remove unnecessary include.
Inline the classes.
Tweaked RandomWELL512a_SSE2::GetUnsigned().

It now has only one return, which seems to help gcc a bit when it's inlined.
Convert from tabs to spaces.
Minor code standard cleanup.
Aligment improvement.

Small performance increase if the RandomWELL512a_SSE2 is properly SSE
aligned and the single value method is used.
Updated the readme.