Fix attr{off,on,set} to use the EncodeAttrs trait.

Had to introduce local wrappers around the foreign nc::* functions in
order to avoid a need to link ncurses in the clients of this crate.
Use EncodeAttrs trait to enable unary attr as arg to attrset.
Added chstr and str output routines: [mv][w]add[ch][n]str.

Refactored code a bit so that API's common to Context and Window are
near eachother.
Refactored attr modification methods into AttrSet trait.
Factored getch/getstr input methods into a GetCh trait.