0.2.1

• Bug fix: Default constructor of pool_allocator can throw.

0.2.0

• Bug fix: Constructor of pool_allocator must make sure that singleton
  is created.
• Define macro SFL_POOL_ALLOCATOR_EXTRA_CHECKS to enable extra checks.
  Extra checks are perfomed only in debug mode (i.e. when macro NDEBUG is
  not defined).

0.1.0

• First release.