Use C++11 regular expressions instead of LibBoost.

Note that current GCC (version 4.7) support is still abysmal, so
we have to wait making this switch. It seems that full regex support
will be implemented in GCC 4.9 [1]. Alternatively, LLVM Clang can
be used with the internal C++ libraries (not default) [2].

[1] http://stackoverflow.com/questions/12530406/is-gcc4-7-buggy-about-regular-expressions
[2] https://bbs.archlinux.org/viewtopic.php?id=162623