Compile user-defined setters in raw JS types to field assignment.

I.e., emit
x.foo = y;
for a method foo defined as
def foo_=(value: T)

(Before this was only done for genuine "setters", i.e.,
accessor methods generated for 'var' definitions.)
Fixed #8: issue when accessing a dynamic.x.

Because Predef provides implicit conversions from Any
to two types with a field named 'x' in Scala 2.10, it
was impossible to access
dynamic.x
for dynamic: js.Dynamic.

This commit explicitly adds accessors for 'x' in
js.Dynamic to work around this issue.