add Thirty360 daycount convention

[lucasprocessi]

No description provided.

[codecov]

Codecov Report

Merging #16 (e15eaec) into master (4f63398) will increase coverage by 0.35%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
+ Coverage   89.03%   89.39%   +0.35%     
==========================================
  Files          11       11              
  Lines         447      462      +15     
==========================================
+ Hits          398      413      +15     
  Misses         49       49              

Impacted Files	Coverage Δ
src/daycount.jl	95.74% <92.30%> (-1.32%)	⬇️
src/types.jl	90.00% <100.00%> (+0.12%)	⬆️
src/methods.jl	97.05% <0.00%> (+1.01%)	⬆️

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