Fixed errors and warnings (clang 3.4)

./dessert.hpp:59:44: error: explicit specialization of 'to_str' in class scope
template<          > static string to_str<timer::time_point>(
const timer::time_point &start ) {
^

./dessert.hpp:58:89: error: invalid operands to binary expression ('stringstream' (aka 'basic_stringstream<char>') and 'const
std::__1::chrono::time_point<std::__1::chrono::steady_clock,
std::__1::chrono::duration<long long, std::__1::ratio<1, 1000000000> > >')
template<typename T> static string to_str( const T &t ) { stringstream ss; return (ss << t) ? ss.str() : "??"; }