tests source and target with differing names

can't believe this was not tested explicitly already, but this catches
an error in the pull-request to support custom data types in postgres.
it made it look like source_table_name and table_name were
interchangeable.
puts source and target options on db connections

really, there should be a source plan and separate target plan, with
only one table name on each object. this would allow for more interesting
column transformations, also. but for now it was a smaller refactoring to put
the source and target knowledge on the db connections.
removes dead code; drys up postgres and mysql
removes more dead code

the sequel issue was fixed long ago. and go away drop_table, you're
drunk - this class is already a delegator.
pushes establishing db connection classes

pass connection options directly into classes, and let them own
connections. set charset in the initializer, and not outside in some
factory
constantizes load data infile sql
Merge pull request #8 from square/da3mon-makes-dbs-know-source-or-target

makes dbs know source or target