Implemented Model#allTables()

Changed to overwrite model files by each test.
Created new sbt tasks

Implemented scalikejdbcGenAll, scalikejdbcGenForce and
scalikejdbcGenAllForce are newly created.
Changed generator srcDir to target/generated_src/
Fixed keyName

Thanks to @xuwei-k
Merge pull request #238 from tkawachi/all-tables

Sbt task to generate models for all tables