Cleanup test warnings, update gitignore

[simonmarty]

Issue #, if available:

Description of changes: Add Maven gitignore, cleanup all IDE warnings.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #28 (50c005b) into master (4af0926) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #28   +/-   ##
=========================================
  Coverage     96.98%   96.98%           
  Complexity       99       99           
=========================================
  Files             6        6           
  Lines           232      232           
  Branches         25       25           
=========================================
  Hits            225      225           
  Misses            5        5           
  Partials          2        2           

Impacted Files	Coverage Δ
...ecretsmanager/caching/cache/SecretCacheObject.java	100.00% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4af0926...50c005b. Read the comment docs.