lib/media_wiki/gateway.rb: Recent MediaWiki versions (>= 1.19?) don't return @apfrom anymore.

This is inherently untestable with FakeMediaWiki and may not work on older MediaWiki versions.
lib/media_wiki/gateway.rb: Fix query continuation.

* Query continuation logic has been extracted into private `iterate_query` method.
* XPath selector for query continue parameter updated to match both `@XXfrom` and `@XXcontinue`.
* Query continue parameter for `usercontribs` changed to `uccontinue`, hence pull request #56 obsolete.
* `count` parameter in `contributions` kept for backwards compatibility, though inconsistent with other methods.

Latest release (0.6.0):

* `list('Media')`: "undefined method `value' for nil:NilClass"
* `category_members('Category:Mass media')`: "MediaWiki::APIError: API error: code 'warning', info 'API warning: Unrecognized parameter: 'apfrom''"
* `backlinks('Media')`: 1214
* `users`: 2870
* `contributions('Lucid')`: "undefined method `value' for nil:NilClass"

Before this commit:

* `list('Media')`: 2373 (fixed)
* `category_members('Category:Mass media')`: "MediaWiki::APIError: API error: code 'warning', info 'API warning: Unrecognized parameter: 'apfrom''" (unchanged)
* `backlinks('Media')`: "MediaWiki::APIError: API error: code 'blbadcontinue', info 'Invalid continue param. You should pass the original value returned by the previous query'" (broken by 25493ab)
* `users`: 500 (broken by 030b81f)
* `contributions('Lucid')`: 5076 (fixed)

After this commit:

* `list('Media')`: 2373 (unchanged)
* `category_members('Category:Mass media')`: 100 (fixed)
* `backlinks('Media')`: 1214 (fixed)
* `users`: 2870 (fixed)
* `contributions('Lucid')`: 5076 (unchanged)
lib/media_wiki/{gateway,utils}.rb: Fix Ruby warnings.

* utils.rb:24: warning: assigned but unused variable - parts
* gateway.rb:115: warning: character class has duplicated range: /\shref="\/wiki\/([\w\(\)_\-\.%\d:,]*)"/
* gateway.rb:346: warning: assigned but unused variable - in_progress
* gateway.rb:508: warning: assigned but unused variable - dummy
* gateway.rb:515: warning: assigned but unused variable - imgs
* gateway.rb:537: warning: assigned but unused variable - dummy
* gateway.rb:609: warning: assigned but unused variable - dummy
* gateway.rb:697: warning: assigned but unused variable - dummy
* gateway.rb:730: warning: assigned but unused variable - dummy
* gateway.rb:755: warning: assigned but unused variable - dummy
* gateway.rb:778: warning: assigned but unused variable - dummy
* gateway.rb:787: warning: assigned but unused variable - dummy
* gateway.rb:795: warning: assigned but unused variable - dummy
* gateway.rb:808: warning: assigned but unused variable - dummy
* gateway.rb:825: warning: assigned but unused variable - dummy
* gateway.rb:843: warning: assigned but unused variable - dummy
* gateway.rb:917: warning: mismatched indentations at 'end' with 'case' at 901
* gateway.rb:960: warning: shadowing outer local variable - e
* gateway.rb:949: warning: assigned but unused variable - e

Ruby versions checked: 2.1.2p95 and 1.9.3p484.
Merge pull request #58 from blackwinter/fix-from-vs-continue

Fix from vs. continue
Merge pull request #60 from blackwinter/fix-ruby-warnings

Fix Ruby warnings.