Fix type error in build

Compilation fails on clang 3.4.1 (and presumably later, tho I haven't tested) with

```
src/reader.cpp:131:59: error: no viable conversion from 'iterator' (aka '__map_iterator<typename __base::iterator>') to 'std::map<std::string, unsigned int>::const_iterator' (aka '__map_const_iterator<typename __base::const_iterator>')
      std::map<std::string, unsigned int>::const_iterator it = counterMap.find("text/html");
                                                          ^    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/v1/map:713:29: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'iterator' (aka '__map_iterator<typename __base::iterator>') to 'const std::__1::__map_const_iterator<std::__1::__tree_const_iterator<std::__1::__value_type<std::__1::basic_string<char>, unsigned int>, std::__1::__tree_node<std::__1::__value_type<std::__1::basic_string<char>, unsigned int>, void *> *, long> > &' for 1st argument
class _LIBCPP_TYPE_VIS_ONLY __map_const_iterator
                            ^
```

because we are not using the right type for the map iterator. As we are using
C++11, let's use `auto` and make compiler set the right type for us.

src/reader.cpp

@@ -128,7 +128,7 @@ namespace kiwix {
     if (counterMap.empty()) {
       counter = this->nsACount;
     } else {
-      std::map<std::string, unsigned int>::const_iterator it = counterMap.find("text/html");
+      auto it = counterMap.find("text/html");
       if (it != counterMap.end())
 	counter = it->second;
     }
@@ -144,9 +144,7 @@ namespace kiwix {
     if (counterMap.empty())
       counter = this->nsICount;
     else {
-      std::map<std::string, unsigned int>::const_iterator it;
-
-      it = counterMap.find("image/jpeg");
+      auto it = counterMap.find("image/jpeg");
       if (it != counterMap.end())
 	counter += it->second;