Adjusted exception mechanism

On Natural string parsing constructor, modified `std::range_error: Non-digit character` condition
WIP #1
WIP #2

Integer modulo needs to be reworked to better suits Rational trimming. More to come soon. :)
Improved Integer modulo operation

Integer now defines modulo as remainder of the Euclidean division, even for negative numbers.
Fixes and Rational trim()

Solved a bug with 1-digit Natural division, which was causing trouble with Integer modulo and with Rational trimming.
Fixes and Rational addition

Fixed a bug on Natural division caused by successive zeros in the dividend, causing trouble to Rational trimming.

Implemented Rational addition.
WIP #3

Arithmetic operations being implemanted. More to come soon. :)
WIP #4

Implemented modulo operation, which needed the `floor()` method. I made this method public because it will certainly be useful.
End of implementation

Added ++ and -- operators overload. Rational is now fully implemented and is ready to get merged into develop.
Merge branch 'feature-rational' into develop