lib/rdoc/markup/to_label.rb (convert): Make labels cross-browser compatible.

Percent-encoded URL fragments prevent Firefox from jumping to their respective anchors. Wikipedia (MediaWiki) does something similar, with `.` instead of `-`.

There's precedent in `RDoc::{Alias,MethodAttr}#html_name` (54dbcf70) and `RDoc::Context::Section#aref` (431baa1b). This breaks backwards-compatibility, though.
Introduce RDoc::RI::Task
Remove copyright notice /cc
https://github.com/rdoc/rdoc/pull/332#discussion_r20754684
Use '.rdoc' for output dir to work with rdoc-tags /cc
https://github.com/rdoc/rdoc/pull/332#discussion_r20754689
Flatten class /cc
https://github.com/rdoc/rdoc/pull/332#discussion_r20754697
Force minitest ~> 4.7 for autotest
Enable warnings in autotest (like rake test)
Allow failures on ruby 1.8.7

This is due to racc incompatibilities with ruby 1.8.7
Add ruby 2.2.0 and head for travis
Merge pull request #332 from zzak/ri-task

Introduce RDoc::RI::Task
Add #332 to History
Document RDoc::RI::Task

Clean up task names
Ignore gem load failure during test startup

This assumes you have loaded minitest yourself.  RubyGems will fail on
the next require or on the constant lookup if you didn't do it right.

Fixes #313
Merge pull request #330 from blackwinter/make-labels-cross-browser-compatible

Make labels cross-browser compatible.
Add #330 to History
Add test for #302

Numbers in word pairs is allowed but not tested in AttributeManager (but
is probably tested elsewhere).
Add missing RDoc::SingleClass#aref_prefix

This was missing and not implemented which caused errors for users that
documented methods via the singleton class.

Fixes #311
Refactor common setup in TestRDocSingleClass
TestRDocSingleClass does not use XrefTestCase
Add #311 to History