add sign_xml and to_pdf utility methods

[rvalyi]

adição de helpers:

• sign_xml que é um metodo de class e usa o erpbrasil.assinatura
• to_pdf que é um método nas instancias dos bindings raiz (como NFe) e serializa em pdf usando https://github.com/Engenere/BrazilFiscalReport ou https://github.com/erpbrasil/erpbrasil.edoc.pdf

o diff global talvez não é muito legível. Podem olhar commit por commit ou ver o arquivo final aqui https://github.com/akretion/nfelib/blob/add-sign-and-pdf/nfelib/__init__.py e ver que foi adicionado os helpers to_pdf e sign_xml.

[codecov]

Codecov Report

Attention: Patch coverage is 68.08511% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 99.07%. Comparing base (1442dfd) to head (52d55ee).
Report is 2 commits behind head on master.

❗ Current head 52d55ee differs from pull request most recent head fde2006. Consider uploading reports for the commit fde2006 to get more accurate results

Files	Patch %	Lines
nfelib/__init__.py	68.08%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
+ Coverage   98.42%   99.07%   +0.64%     
==========================================
  Files         222      222              
  Lines       10983    11020      +37     
==========================================
+ Hits        10810    10918     +108     
+ Misses        173      102      -71     

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