Port if_then_else to Parser.

Ported cops: IfWithSemicolon, MultilineIfThen, OneLineConditional.
Port if_then_else cops to AST::Processor.
Change favor_unless_over_negated_if according to @whitequark's comment.
Port favor_unless_over_negated_if to AST::Processor.
Merge pull request #189 from jonas054/port_if_then_else

Port if_then_else to Parser.
Allow failures on Rubinius
Merge pull request #190 from jonas054/port_favor_unless_over_negated_if

Port favor unless over negated if
Port SingleLineMethods to AST::Processor
Port Loop to AST::Processor
Add missing super calls
Port a few more cops to AST::Processor
Port SpaceAfterControlKeyword to Parser.
Merge pull request #192 from jonas054/port_space_after_ctrl_kw

Port SpaceAfterControlKeyword to Parser.
Fix --only option.

Only the selected cop was set up with configuration, so if one cop
depended on the configuration of another, it would crash when called
with --only because LineLength.config (for example) was nil.
Merge pull request #194 from jonas054/fix_only_option

Fix --only option.
Migrate to parser-2.0.0.beta1.

Perform name changes source_map to location, src to loc, to_source
to source, cdecl to casgn, and remove cvdecl.
Use the new parse_with_comments() method in Parser.
Port AsciiComments and LeadingCommentSpace to Parser.
Merge pull request #196 from jonas054/port_ascii_comments

Port ascii comments and leading_comment_space
Port more cops to AST::Processor.

The remaining really easy ones.