prettify test_cpp_double_float_constructors.cpp
skip known false negatives while testing string streaming
fix bug in construction from other cpp_double_float<> objects
remove exponent scaling in string conversion
add rigorous test for comparision operators
add std::numeric_limits<> specialization for cpp_double_float
fix subtle bug in split()
fix bugs in constructor
replace abs with fabs as suggested by Chris
add tests for floating-point constructors in test_cpp_constructors.cpp
add specialized random number generator for testing I/O and comparision operators