add operators == (attribute_value, std::string)
fixup 'add operators == (attribute_value, std::string)'

we need to keep the code form using these functions in case of c["a"] ==
0
add operators !=(attribute_value, std::string)
fixup use of operator !=(attribute_value, const char*)

since we don't use t_string's == operator anymore in this case, c["a"]
!= "" will evaluate to true in case of "a" beeing not existent. To get
the desired behaviour we need to use the .empty() method.

I also add an assert in map_generation for a possible segfault that i
noticed during fixing this.
fix unit(_type)::resistance_filter_matches

fix use of operator == (atribute_value, char*)
extend attibute_value unit test
Merge pull request #226 from gfgtdf/config_comparision

add operators ==,!= (attribute_value, std::string)

previously comparisions like c["a"] = "b" were evaluated by 
inline bool operator==(const std::string &a, const t_string &b) 
by casting atribute_value to std::string, and const char * to t_string

we fix this by adding explicit equality operator for these cases.

Not creating t_string also results in significant preformance improvements in some cases.