Add postgresql backend

[mattyb]

This PR creates a PostgreSQL backend for memoise

[codecov-io]

Codecov Report

Merging #82 into master will decrease coverage by 8.5%. The diff coverage is 89.69%.

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
- Coverage   77.37%   68.86%   -8.51%     
==========================================
  Files           5        6       +1     
  Lines         221      318      +97     
==========================================
+ Hits          171      219      +48     
- Misses         50       99      +49

Impacted Files	Coverage Δ
R/cache_postgresql.R	89.69% <89.69%> (ø)
R/cache_s3.R	0% <0%> (-100%)	:arrow_down:

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[jimhester]

Is there a reason you used RPostgreSQL for this instead of RPostgres? Also rather than a postgres specific database maybe it would make more sense to use the odbc package, so any odbc-compliant database could be used?