Fix Build Tests

[sappenin]

• Remove gradle in favor of maven.
• Update pom for Nexus staging and deploy
• Fix test coverage and artifact generation
• Fix Checkstyle checks.
• Remove HexDump.java

[codecov[bot]]

Codecov Report

Merging #67 into development will increase coverage by 37.65%. The diff coverage is n/a.

@@                Coverage Diff                 @@
##             development      #67       +/-   ##
==================================================
+ Coverage          37.89%   75.55%   +37.65%     
- Complexity            97      197      +100     
==================================================
  Files                 24       24               
  Lines                863      863               
  Branches              86       86               
==================================================
+ Hits                 327      652      +325     
+ Misses               507      151      -356     
- Partials              29       60       +31

Impacted Files	Coverage Δ	Complexity Δ
...rg/interledger/cryptoconditions/ConditionType.java	79.72% <0%> (+4.05%)	26% <0%> (+2%)	:arrow_up:
...rg/interledger/cryptoconditions/ConditionBase.java	56.89% <0%> (+6.89%)	8% <0%> (+2%)	:arrow_up:
...ger/cryptoconditions/types/RsaSha256Condition.java	73.91% <0%> (+8.69%)	5% <0%> (+1%)	:arrow_up:
...edger/cryptoconditions/uri/CryptoConditionUri.java	57.77% <0%> (+8.88%)	9% <0%> (+4%)	:arrow_up:
...cryptoconditions/types/Ed25519Sha256Condition.java	89.47% <0%> (+10.52%)	5% <0%> (+1%)	:arrow_up:
.../cryptoconditions/types/PrefixSha256Condition.java	93.54% <0%> (+12.9%)	8% <0%> (+2%)	:arrow_up:
...yptoconditions/types/ThresholdSha256Condition.java	96.15% <0%> (+28.84%)	16% <0%> (+7%)	:arrow_up:
...terledger/cryptoconditions/der/DerInputStream.java	54.83% <0%> (+54.83%)	11% <0%> (+11%)	:arrow_up:
...r/cryptoconditions/types/RsaSha256Fulfillment.java	70% <0%> (+67.5%)	8% <0%> (+7%)	:arrow_up:
...yptoconditions/types/Ed25519Sha256Fulfillment.java	71.11% <0%> (+71.11%)	10% <0%> (+10%)	:arrow_up:
... and 5 more

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[sappenin]

Cancelling this PR because it's incorporated into #68.