Fix boolean parsing on postgres

[gi0baro]

As discussed on the group: https://groups.google.com/forum/#!topic/web2py-developers/m2MV5Y12lhM

This:

• puts back the parsing of multiple boolean types in the base parser
• adds a postgres3 adapter which uses the BOOLEAN type of PostgreSQL for boolean fields

I removed the tests for postgres2 adapter since is implicitly tested by postgres3 as it inherits from postgres2.

[codecov-io]

Current coverage is 67.15%

Merging #373 into master will increase coverage by 0.01%

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  Files            69         69          
  Lines          8796       8824    +28   
  Methods           0          0          
  Messages          0          0          
  Branches       1912       1917     +5   
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+ Hits           5906       5926    +20   
- Misses         2379       2387     +8   
  Partials        511        511          

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