[cleanslate] Access variables and direct variables.

- Extend javadoc.
[cleanslate] Temporarily remove visiting unexisting types.
[cleanslate] Getting and setting values.
[cleanslate] More query expressions.
[cleanslate] Dependency management.
[cleanslate] Rename identifiers to comply with coding style.
[cleanslate] ignore target directory.
[cleanslate] Correct javadoc syntax.
[cleanslate] test contract of QueryExpression.
[cleanslate] test QueryExpression variable accessors.
[cleanslate] Test visitor.
[cleanslate] test Literal.
[cleanslate] ignore target.
[cleanslate] Use vim spell check.
[cleanslate] javadoc.
[cleanslate] derived interfaces can narrow returned collections.

- QueryExpression specifies the return type of subexpressions, variables, and directVariables as "? extends …".
[cleanslate] [WIP] QuerryTerm concept.
[cleanslate] Divide query concepts into TokenExpressions.
[cleanslate] TokenExpressions and TokenVariables.
[cleanslate] Literal ISA TokenExpression.