#142 Fix no source was set to literal block in a bullet list

[Takeshi KOMIYA]

In docutils-0.14, it seems a literal_block node in a bullet list does not have source attributes.
The attached patch sets the source information to the literal_block node.

Could you review this please?

Note: it seems there are no test cases for source and lines. So I simply added the directive code. Please let me know if any good place to do that.

Thanks,

[Günter Milde]

Thank you for the patch.
Committed in [r8219].