Split up into ceylon.ast and ceylon.ast.redhat

As discussed in #6, the main ceylon.ast module will not be coupled to
the Ceylon compiler; the transition from a ceylon.ast AST to a
com.redhat.ceylon.typechecker AST will be implemented in the
ceylon.ast.redhat module (the name of which may change – discuss in #7).

Closes #7 for now.
Add documentation to ceylon.ast, ceylon.ast.redhat
Add Node class