Add support for RFC 2631

[russhousley]

Add module and test for RFC 2631.

[codecov-io]

Codecov Report

Merging #101 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #101      +/-   ##
==========================================
+ Coverage   99.35%   99.35%   +<.01%     
==========================================
  Files          89       90       +1     
  Lines        5766     5774       +8     
==========================================
+ Hits         5729     5737       +8     
  Misses         37       37

Impacted Files	Coverage Δ
pyasn1_modules/rfc2631.py	100% <100%> (ø)

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[etingof]

Thank you!