modifies OutputType enum..removes string field because values are producible without that field; fixes #81
fixes a bug on the signature of null literals

If an expression has an empty signature, an exception is thrown in CtElement.equals.
This is the case for null literals.
The solution is to improve the signature of null literals.
feat #63 Refactors ImportScanner to get imports for a class

Create a new class for the old inner class ImportScanner,
CtImportScanner, and his new interface, CtImport, to have a
contract to retrieve all imports for a CtSimpleType element
(represent a class, an interface or an annotation). And
add a new test case to check if we are all useful imports.
Remove unused classes.
Remove unused imports.
Improve type checking.
Add some @SuppressWarnings annotations.
Merge pull request #90 from monperrus/bug-equal-signature-null
Merge pull request #78 from GerardPaligot/feat_63
Merge pull request #85 from boeserwolf91/output_type
Merge pull request #92 from seintur/code_cleanup