Refactor database config normalization

Two minor changes to configuration_spec:
- replaced adapter_name with adaptername in dummy environments;
  underscore is not valid in URL schema
- URLencoded commas in search_path
Don't fail initialization if config/database.yml is missing
Make specifying jdbc adapters specifically work
Support for Ruby 1.8 and ActionPack 3.2
Make rubocop happy
Emit URLs that are acceptable for JDBC
Add respond_to_missing? to DbConfig
Wrap pretend_file_not_exists in a module
Check for specific keys in db config to recognize missing config

Sometimes stuff (eg. search path) is set from elsewhere. But a config
with no adapter nor url is unlikely to be valid.

Note this is purely cosmetic change: it would fail either way, but
this allows for a better error message.
Merge pull request #78 from dividedmind/support-no-database-yml

Support running without database.yml