added parallel hash build

removed one recurring vector.size call

further performance tuning by avoiding calls to vector.size()

fixed an error in hash_table_test

added some comments

added hash_build tests

Adding Single Key Hash Building

Adding HashJoin with Single Keys for Values

removed google profiler.h from header

Automatically Selecting the Appropriate HashMap Type

Updating the MergeHashTable with appropriate hash table choice

Bugfix in Interface
Merge branch 'feature/single_value_join' of github.com:hyrise/hyrise into feature/single_value_join

Conflicts:
	src/bin/units_access/hash_build.cpp
	src/lib/access/HashBuild.cpp
	src/lib/access/MergeHashTables.cpp
	src/lib/storage/HashTable.h