Enable C++11

Now that we have a branch with a more recent gcc version we can enable
C++11 support.

This enables some more powerful and useful C++ features, like variadic
templates, rvalue refs / move constructors (for more efficient code -
String class already supports these), constexpr (to allow e.g. function
calls as constant values), foreach-style loop, enumeration classes,
static assertions. See http://en.wikipedia.org/wiki/C%2B%2B11 for
details.

These features are mostly useful for more advanced users and library
authors, but there is a lot in there that can make a significant
difference to code efficiency or conciseness.

Enabling C++11 should be completely backwards compatible.