add option to change GENERATION_SUFFIX

see https://github.com/excilys/androidannotations/issues/1209

this allows to set the `generationSuffix` via options. at the moment the same valule is used for all generated stuff (classes, fields, methods) where it before was `_`. But for a first look i already opened the PR.
We have to validate the suffix here, because the user can pass a string which is not valid in Java identifiers.