Add automap support for concrete classes

[cosmin-ionita]

Hello, @davidmoten!

Thank you for your work on this project, it's very useful and it definitely helps a lot of developers.

I noticed that auto-map support for concrete classes was dropped on this project (comparing to rxjava-jdbc project), for the reasons you mentioned on issue #15, which I completely understand.

However, it is an important feature that is really needed, mostly for the ones who migrate from rxjava-jdbc, which need to update their models to be compatible with this new project.

In alignment to your expressed intention (in the above mentioned issue) to add back that functionality, I decided to take some time to explore the code and see if I can add it back on (and this PR is the result of that effort).

I mainly took the code from the rxjava-jdbc and I integrated it here, as best as I could, and I also added a small unit test to see if it works as expected.

Thank you and let me know what you think about it!

[codecov-io]

Codecov Report

Merging #53 into master will decrease coverage by 0.78%.
The diff coverage is 28.88%.

@@             Coverage Diff              @@
##             master      #53      +/-   ##
============================================
- Coverage     77.64%   76.86%   -0.79%     
- Complexity      669      675       +6     
============================================
  Files            73       73              
  Lines          2747     2792      +45     
  Branches        240      249       +9     
============================================
+ Hits           2133     2146      +13     
- Misses          508      540      +32     
  Partials        106      106              

Impacted Files	Coverage Δ	Complexity Δ
...dbc/src/main/java/org/davidmoten/rx/jdbc/Util.java	71.22% <28.88%> (-3.78%)	134.00 <6.00> (+6.00)	⬇️

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