Consistent face handling with defface.

Turned merlin-compilation-warning-face and merlin-compilation-error-face into
actual faces. Thus they will be easier to customize and be consistent with
merlin-locked-face and merlin-type-face.

Also fixed defface for merlin-type-face by removing superfluous quote char so
now the customize interface displays the proper editor instead of just an
s-expression.
Merge branch 'master' of https://github.com/sheijk/merlin into sheijk-master