Add less than or equal to for Decimals

[nicoleepp]

Closes #46

[coveralls]

Pull Request Test Coverage Report for Build 118

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.2%) to 96.262%

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Totals
Change from base Build 116:	1.2%
Covered Lines:	103
Relevant Lines:	107

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💛 - Coveralls

[codecov-io]

Codecov Report

Merging #47 into master will increase coverage by 1.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
+ Coverage   95.28%   96.29%   +1.01%     
==========================================
  Files           6        6              
  Lines         106      108       +2     
==========================================
+ Hits          101      104       +3     
+ Misses          5        4       -1

Impacted Files	Coverage Δ
src/Decimals.jl	100% <ø> (ø)	⬆️
src/equals.jl	100% <100%> (ø)	⬆️
src/arithmetic.jl	81.25% <0%> (+1.25%)	⬆️
src/decimal.jl	100% <0%> (+2.63%)	⬆️

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[mattBrzezinski]

I don't understand why >= is working, but I feel like be explicit about it here would be beneficial.

[fchorney]

I don't understand why >= is working, but I feel like be explicit about it here would be beneficial.

in julia x >= y is defined as y <= x so you only need to define the one
>=(x, y) = (y <= x)