[WIP] Add sqlite3 support

Add sqlite3 support. 

This PR is almost all good, however I believe that one of the (existing) tests doesn't correctly test what it is supposed to be testing. 

running `DB=sqlite be rspec` multiple times will result in one particular test passing or failing randomly. That test is `DoubleEntry::Locking should allow multiple threads to lock at the same time` @ spec/double_entry/locking_spec.rb:136

I don't believe that it should *ever* pass with sqlite3, as it is not possible for more than one thread to ever lock  rows as sqlite3 does whole-db locking. 

I'm not sure if I'll ever be able to get the jackhammer script to pass, due to sqlite locking.