Added Abs(), fixed bugs in comparison

• Added a static method .Abs(BigFraction v) that returns an absolute value
• Fixed a bug in comparison giving wring results sometimes if denominator is negative

BigFraction is now a readonly struct

Also removed operator %, as it was implemented it didn't make much sense.

Optimizations

FromDouble(...) : ~1.5 % avg improvement
ToDecimal(...) : 15-40 % improvement in certain cases (when |Numerator| or |Denominator| > decimal.MaxValue)
(int,int) and (long,long) constructors + more idiomatic checks for zero
2x-2.5x speed up for operations where one of the operands is a BigInteger
More comprehensive testing

Pre-defined popular constants (-1, 0, 1)

Merge pull request #8 from aprismatic/one_n_zero

Added static pre-defined values for 1, 0, -1
Bumped xunit version

Fields for optimization

Merge pull request #7 from aprismatic/sign

Fields for optimization

Release NuGet Packages

0.1.2

Release nuget package as artifact