Remove hand written conversion code and replace with stdlib features.

With C++11 the stdlib gains:

- `std::stoi` function family to convert from `std::string` to integral type

- `std::to_string` to convert from number types to `std::string`

The only reason for hand-writing the conversion code therefore is
performance. I benchmarked an `osrm-extract` with the hand-written code
against one with the stdlib conversion features and could not find any
significant difference (we switch back and forth between C++ and Lua,
shaving off a few us in conversion doesn't gain us much).

Formatting arithmetic types in the default format with given precision
requires streams, but is doable in a few lines of idiomatic stdlib code.

For this, there is now the following function template available:

    template <Arithmetic T, int Precision = 6>
    inline std::string to_string_with_precision(const T);

that requires integral or floating point types and returns a formatted
string in the defaukt format with the given precision applied.

In addition this completely rips out Boost.Spirit from the `casts.hpp`
header, resulting in faster compile times.

Boom!

References:

- http://en.cppreference.com/w/cpp/string/basic_string/stol
- http://en.cppreference.com/w/cpp/string/basic_string/to_string
- http://www.kumobius.com/2013/08/c-string-to-int/

server/http/reply.cpp

@@ -27,7 +27,7 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #include "reply.hpp"
 
-#include "../../util/cast.hpp"
+#include <string>
 
 namespace http
 {
@@ -48,7 +48,7 @@ void reply::set_size(const std::size_t size)
     {
         if ("Content-Length" == h.name)
         {
-            h.value = cast::integral_to_string(size);
+            h.value = std::to_string(size);
         }
     }
 }
@@ -95,7 +95,7 @@ reply reply::stock_reply(const reply::status_type status)
     const std::string status_string = reply.status_to_string(status);
     reply.content.insert(reply.content.end(), status_string.begin(), status_string.end());
     reply.headers.emplace_back("Access-Control-Allow-Origin", "*");
-    reply.headers.emplace_back("Content-Length", cast::integral_to_string(reply.content.size()));
+    reply.headers.emplace_back("Content-Length", std::to_string(reply.content.size()));
     reply.headers.emplace_back("Content-Type", "text/html");
     return reply;
 }