Automatically use Redshift 4.0 or 4.1 JDBC driver, depending on which is installed

[JoshRosen]

This patch refactors the driver-loading code so that it automatically uses either the Redshift JDBC 4.0 or 4.1 drivers, depending on which is installed. I also added support for automatically supplying an appropriate default JDBC driver class name when the postgres:// subprotocol is used.

I tested all of the error-handling corner-cases manually.

Fixes #83.

[codecov-io]

Current coverage is 94.84%

Merging #90 into master will increase coverage by +0.03% as of 4835231

@@            master     #90   diff @@
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  Files           11      11       
  Stmts          444     446     +2
  Branches       107     109     +2
  Methods          0       0       
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+ Hit            421     423     +2
  Partial          0       0       
  Missed          23      23       

Review entire Coverage Diff as of 4835231

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[marmbrus]

LGTM