Bug correction (#31), set a moved hash map in a valid state so that i…

…t can still be used even after a move (#32)

* Bug correction, the hash table is now in a valid state after a move (#31). This commit also allows the hash table to have a bucket count equals to 0.

* Rename m_load_threshold to m_max_load_threshold_rehash and m_min_load_factor_rehash_threshold to m_min_load_threshold_rehash.

* Add clear() method in growth policies classes + minor refactoring.

* Add tests on using a moved hopsctoch_map.

* Remove unused MIN_BUCKETS_SIZE variable in mod_growth_policy.