Patch SBJsonStreamParser for integral values.

SBJsonStreamParser parses an integer into an NSDecimalNumber. With some
large values, such as Twitter tweet ids, this results in a rounding
error. This pull request makes a one line change to the parser to parse
into a longLongNumber instead of a decimal number. It also modifies the
sample app to use the Apple social media APIs and accounts. The problem
can be demonstrated by uncommenting the original line in the
SBJsonStreamParser file. Then the app will start logging tweets that
have incorrect values for tweet ids.
Better fix for #171 - support full range of unsigned long long as proper integer type

Integers are now always parsed into `long long` or `unsigned long long` if they fit.
Move to less surprising handling of floating point numbers - fixes #113, #128 & #164

This removes the last trace of NSDecimalNumber, which has some surprising behaviour. It allows more precision but less range than double. This required:

* Removing tests for real numbers that cannot be represented accurately using IEEE floating point.
* Avoid rounding issues in tests when all we want is to make sure exponent is handled.

For clarity this also renamed tests for (positive and negative) zero for real numbers.
Fix for #168 - name of directory containing classes in README