Custom memory handler fixes

- Make all custom memory handler be defined through a single
  preprocessor symbol to prevent incorrect mismatches.

- Reintroduce BOOST_SIMD_MEMORY_NO_BUILTINS to deactivate
  buitl-in aligned memory handlers without requiring custom
  handlers. This was erroneously removed in a previous commit.

- Amend doc to reflect new state of things.
Merge pull request #678 from MetaScale/fix_builtin_memory

Custom memory handler fixes