Check for presence and functionality of <random>

Implement compile/run test for presence and basic functionality of
C++11 <random> header. If test passes, set CMake variable
HAS_CXX11_RANDOM to true (false otherwise). Document variable in
CheckCXX11Features module and link to corresponding FNAL cpp0x
preprocessor symbol in README.

NB: test does not check for presence of all generators and
distributions. It is not clear yet if this is required.
Add cmakedefine for HAS_CXX11_RANDOM in example header
Fix typo in check of default template arguments

Call to cxx11_check_feature for default template arguments contains
a typo, resulting in wrong CMake variable getting set. Resolve typo
so that call uses correctly spelt and documentated variable name.