Add support to P2SH output signatures

[alejandro-isaza]

Don't merge, missing test.

[codecov-io]

Codecov Report

Merging #48 into master will decrease coverage by 0.51%.
The diff coverage is 37.5%.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
- Coverage   80.94%   80.43%   -0.52%     
==========================================
  Files          72       72              
  Lines        2892     2903      +11     
==========================================
- Hits         2341     2335       -6     
- Misses        551      568      +17

Impacted Files	Coverage Δ
Sources/Bitcoin/BitcoinTransactionSigner.swift	55.31% <37.5%> (-33.57%)	⬇️

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[hewigovens]

let's close this and working on #49