first commit on branch issue22
use seperate db apis
update
add __all__
add execute method
add leaf. node, sql
add Field, ForeignKey, PrimaryKey
fix typo; add functions
normalize db key of configs
add psql!
add select_db
add conn_is_alive for Pycospg2
current add tests for all dbapis: fuck postgres
fix tests/READMe
typos
update docs
remove Node, remove fullname; add Alias
add Model
add alias2sql
add expr2sql