* Add opt-in c++11 stream insertable check.

To opt-in, define CATCH_CONFIG_CPP11_STREAM_INSERTABLE_CHECK.

Opt-in fixes #872 and should fix #757 as well.

When using C++11, comparison operators are already templated to take
anything that can be explicitly converted to double, but constructor
took only doubles. This lead to warnings when an `Approx` was
constructed from floats, which was problematic for some users.

Since just adding float constructor would be a large breaking change, as
suddenly `Approx( 1 )` would become ambiguous, I added a templated
constructor that will take anything that is explicitly convertible to
double. This has the added benefit of allowing constructing `Approx`
instances from instances of strong typedefs, ie allowing
`calculated_temp == Approx( known_temp)`.

Closes #873

Github wouldn't let me push to the PR, probably because it was made from
fork of a fork...

Unexpected exceptions no longer cause abort and there should be no more
potential for false negatives.

The trade-off now is that exceptions are no longer translated.

`ResultBuilder`s destructor now checks for exceptions, preventing false
negatives. The speedup should remain the same give or take a tiny bit.

This is another warning that follows test macros, making it painful to
suppress without leaking outside. Luckily clang's `_Pragma`
implementation works.

Should fix #308