Add Sqlite3 adapter, analogues to the postgres one.
Support multiple backends in test, to verify Sqlite3 is working.
To try it out use DM_ENV_NAME=sqlite3 rspec
Prefer Hash#fetch when a missing key should return a default value.
In specs : rename DM_REPO_ENV to DM_ADAPTER.
Rename the in_memory adapter used for testing from :test to :in_memory
Merge pull request #36 from arnebrasseur/sqlite3_adapter

Add Sqlite3 adapter, analogues to the postgres one.