Merge pull request #25 from lysiszegerman/master

Generic SQL support for source
Return useful error message on missing SQL adapter

SQL file in configuration renamed to database.
Should the generated example file have host+user+password+query?
Depend on newer asetus version

We are doing this in sql.rb

:user => @cfg.sql.user?
:password => @cfg.sql.password?

As 'asetus' will accept arbitrary chain

@cfg.sql.one.two.three

it is clear that 'one' or 'two' cannot _ever_ return nil/false,
otherwise you cannot create 'three'.

So if there isn't value of one/two, instance of 'asetus' is returned,
upon which we can build two, and then tree'.

one?, two?, three? version existed in old asetus, as strict boolean test

@cfg.sql.one? would return true, false or nil

In newer version, instead of returning true, it returns actual value.
bump up gemspec

- actually start to follow semantic version
      - a.b.c   a == major, b == minor c == patch
      - c only for bug-fixes etc
      - b for non-breaking new features
      - a for breaking new featires
      - a == 0 is special, can break anywhere
      - http://semver.org

- add more specific version restrictions for gems