Add: ht/common.h with debug helper macro
Added ht/common.h header
Added debug output: ht_init()
Added debug output: ht_destroy()
Added debug output: ht_del()
Added debug output: ht_find()
Added debug output: ht_ndel()
Added debug output: ht_insert()
Merge pull request #221 from matthiasbeyer/add-debug_output_ht

Add debug output for hashtable implementation