Improve checks for string and number formulas

Basically, checking for a number formula is more reliable than looking
for a string formula.

The main requisite for a number formula is:

- Is an NSNumber
- Is a NSString that contains 0123456789
Improve tests
Update NSString-HYPFormula.podspec
Show string formulas
Show results
Improve is number formula, number tests
Merge pull request #14 from hyperoslo/improve/is-number-formula

Improve/is number formula