Add legal to source distribution, update copyright date

[rly]

• Add Legal.txt to manifest to fix The release tar does not include the hdmf-common-schema sub module #195
• Update copyright date to 2020

[codecov]

Codecov Report

Merging #232 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #232   +/-   ##
=======================================
  Coverage   70.48%   70.48%           
=======================================
  Files          32       32           
  Lines        5932     5932           
  Branches     1390     1390           
=======================================
  Hits         4181     4181           
  Misses       1320     1320           
  Partials      431      431

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