Skip nil tokens in RDoc::TokenStream#tokens_to_s.

For reasons I don't know, `token_stream` sometimes contain `nil` elements.
This is handled in `RDoc::TokenStream.to_html` but not in
`RDoc::TokenStream#tokens_to_s`. This adds handling to
`RDoc::TokenStream#tokens_to_s`.
Make +X::Y+ be treated as typewriter text, same as <tt>X::Y</tt> (Fixes #319)

We could use a more specific regexp that only matches :: and not
just : inside the string, but I didn't think it was worth the
additional complexity.
Fix lexing of %w'1 2', %r'foo(bar)?' and similar syntax (Fixes #315)
fix pp for method_attr alias

Pretty print fails when the method is loaded from the store and has an alias.
add test for pp on method loaded from store
Merge pull request #324 from Elffers/pretty_print

fix pp for method_attr alias
Add #324 to History
Merge pull request #323 from jeremyevans/lex_wr_quote

Fix lexing of %w'1 2', %r'foo(bar)?' and similar syntax (Fixes #315)
Add #315, #323 to History
Merge pull request #322 from jeremyevans/tt_constants

Make +X::Y+ be treated as typewriter text, same as <tt>X::Y</tt> (Fixes #319)
Add #319, #322 to History
Merge pull request #318 from calleerlandsson/fix-tokens-to-s

Skip nil tokens in RDoc::TokenStream#tokens_to_s.
Add #318 to History
Fix Darkfish footer links.
Merge pull request #328 from blackwinter/fix-darkfish-footer-links

[ci skip] Fix Darkfish and rubyforge footer links.
Add History for #328