FEAT - SHA-256 checksumming

[carlwilson]

• added SHA-256 support to Checksummer class;
• added SHA-256 instance to ChecksumType enum;
• updated comments to reflect calculation of SHA-256; and
• ignore README files when testing.

Closes #104 and includes work done in #386 thanks to @escowles

[escowles]

👍 This looks good to me, thanks for resurrecting this @carlwilson !

[tledoux]

Looks good to me too.

[carlwilson]

It still requires me to come up with a test patch to add or ignore the new checksums on travis.

[codecov]

Codecov Report

Merging #497 into integration will decrease coverage by 0.16%.
The diff coverage is 52.17%.

@@                Coverage Diff                @@
##             integration     #497      +/-   ##
=================================================
- Coverage          49.69%   49.53%   -0.17%     
+ Complexity           987      985       -2     
=================================================
  Files                 55       55              
  Lines               7750     7767      +17     
  Branches            1371     1373       +2     
=================================================
- Hits                3851     3847       -4     
- Misses              3435     3450      +15     
- Partials             464      470       +6

Impacted Files	Coverage Δ	Complexity Δ
...main/java/edu/harvard/hul/ois/jhove/JhoveBase.java	41.52% <0%> (-0.27%)	41 <0> (ø)
...n/java/edu/harvard/hul/ois/jhove/ChecksumType.java	100% <100%> (ø)	3 <0> (ø)	⬇️
...a/edu/harvard/hul/ois/jhove/NisoImageMetadata.java	75.64% <100%> (ø)	182 <0> (ø)	⬇️
...ain/java/edu/harvard/hul/ois/jhove/ModuleBase.java	50.34% <28.57%> (-0.36%)	70 <0> (ø)
...in/java/edu/harvard/hul/ois/jhove/Checksummer.java	56.92% <57.57%> (-10.88%)	13 <9> (ø)
...c/main/java/edu/harvard/hul/ois/jhove/RepInfo.java	53.07% <0%> (-3.08%)	36% <0%> (-2%)

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