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 should also be sure the string is not empty (zero-lenth), because it will result in duplicate class names.