Added more floating point tests.

The first demonstrates a bug that I will soon fix.

git-svn-id: https://svn.code.sf.net/p/jsoncpp/code/trunk/jsoncpp@271 1f120ed1-78a5-a849-adca-83f0a9e25bb6
Replaced the complex implementation of valueToString(double).

The previous one was confusing and prone to buffer overflows, and didn't
work correctly with 16-decimal-digit numbers. The new one simply uses
snprintf with a standard format string.

The major change is that we don't always print a decimal point now.
Fortunately, JSON doesn't distinguish between integers and reals.

git-svn-id: https://svn.code.sf.net/p/jsoncpp/code/trunk/jsoncpp@272 1f120ed1-78a5-a849-adca-83f0a9e25bb6
Updated two calls to sprintf that I missed in r269.

git-svn-id: https://svn.code.sf.net/p/jsoncpp/code/trunk/jsoncpp@273 1f120ed1-78a5-a849-adca-83f0a9e25bb6