v0.6.3

• Fix issue #26, raise the maximum possible size of the hash table when using the prime_growth_policy on a 64-bit platform.
• Fix issue #31, when min_load_factor() > 0, the clear() method will also reset the bucket_count of the hash table to 0.
• Fix shrink when min_load_factor is set and a range erase with end() as last is called. The m_try_skrink_on_next_insert was not correctly set.
• Fix issue #33, the value function of a const iterator can now be called and returns a mutable reference to the underlying value_type.