introduce rubocop?!

This idea came to my mind while talking over style in #854 as @jasonrclark mentioned the [ruby style guide](https://github.com/bbatsov/ruby-style-guide). There is [rubocop](https://github.com/bbatsov/rubocop) automatically checking all those style guidelines.

Personally I don't personally agree with all those style guidelines (e.g. I like to have no spaces around `{`, `}` in one line blocks as that looks compact), a lot of the style guides are good.

This is meant for discussion about if we want to add it as a dependency and use it to check our style.

If we disagree with specific checks, we can turn them off.

Right now our result is quite high:

```ruby
tobi@speedy:~/github/shoes4$ rubocop lib/ -f o

181   Style/Documentation
168   Style/SpaceInsideParens
153   Metrics/LineLength
136   Style/StringLiterals
124   Style/SpaceInsideBlockBraces
110   Style/SpaceAroundOperators
102   Lint/UnusedMethodArgument
65    Style/CaseIndentation
57    Style/SpaceBeforeBlockBraces
56    Style/EmptyLinesAroundBody
54    Metrics/MethodLength
54    Style/HashSyntax
53    Style/MethodDefParentheses
50    Style/SpaceInsideHashLiteralBraces
39    Style/SpaceAroundEqualsInParameterDefault
35    Style/AndOr
32    Style/EmptyLinesAroundAccessModifier
26    Style/RedundantSelf
23    Style/TrailingWhitespace
22    Style/LeadingCommentSpace
21    Lint/AmbiguousOperator
21    Metrics/ParameterLists
19    Style/PerlBackrefs
19    Style/TrivialAccessors
18    Style/TrailingBlankLines
17    Style/DotPosition
17    Style/EmptyLines
16    Style/GuardClause
14    Style/SignalException
13    Metrics/CyclomaticComplexity
12    Lint/UnusedBlockArgument
12    Lint/UselessAssignment
12    Style/Semicolon
12    Style/WhenThen
11    Metrics/ClassLength
11    Style/AlignParameters
10    Style/MethodCallParentheses
9     Metrics/BlockNesting
9     Metrics/PerceivedComplexity
9     Style/AccessorMethodName
9     Style/PercentLiteralDelimiters
8     Style/IfUnlessModifier
8     Style/MethodName
8     Style/SpaceAfterComma
8     Style/TrailingComma
7     Style/Not
6     Style/Blocks
6     Style/EndOfLine
6     Style/IndentHash
6     Style/PredicateName
5     Style/AlignArray
5     Style/CharacterLiteral
5     Style/FormatString
5     Style/LineEndConcatenation
5     Style/Proc
5     Style/SingleSpaceBeforeFirstArg
4     Style/ClassAndModuleChildren
4     Style/GlobalVars
4     Style/NilComparison
4     Style/RedundantReturn
4     Style/RegexpLiteral
4     Style/SpaceBeforeSemicolon
4     Style/SpaceInsideBrackets
3     Lint/ShadowingOuterLocalVariable
3     Lint/StringConversionInInterpolation
3     Style/CollectionMethods
3     Style/ColonMethodCall
3     Style/CommentAnnotation
3     Style/DeprecatedHashMethods
3     Style/EvenOdd
3     Style/IndentationWidth
3     Style/ParenthesesAroundCondition
3     Style/SpaceAfterColon
3     Style/UnlessElse
2     Lint/DeprecatedClassMethods
2     Lint/Eval
2     Lint/RequireParentheses
2     Lint/UselessAccessModifier
2     Style/AccessModifierIndentation
2     Style/BracesAroundHashParameters
2     Style/CommentIndentation
2     Style/EmptyLineBetweenDefs
2     Style/IndentationConsistency
2     Style/NegatedIf
2     Style/NestedTernaryOperator
2     Style/SelfAssignment
2     Style/SpaceAfterSemicolon
1     Lint/BlockAlignment
1     Lint/ConditionPosition
1     Lint/HandleExceptions
1     Lint/RescueException
1     Lint/UnderscorePrefixedVariableName
1     Style/AsciiComments
1     Style/BarePercentLiterals
1     Style/BlockEndNewline
1     Style/ClassCheck
1     Style/ConstantName
1     Style/EmptyLiteral
1     Style/MultilineBlockChain
1     Style/MultilineBlockLayout
1     Style/NegatedWhile
1     Style/NonNilCheck
1     Style/OneLineConditional
1     Style/OpMethod
1     Style/RedundantBegin
1     Style/SingleLineBlockParams
1     Style/SpaceBeforeComment
1     Style/VariableInterpolation
1     Style/WhileUntilDo
1     Style/WhileUntilModifier
1     Style/WordArray
--
2035  Total
```

You can see the result of running `rubocop lib/` in [this gist](https://gist.github.com/PragTob/b3376f460d5077e288b7)

A lot of the checks really to help, e.g. sometimes we have spaces inside method definition argument parentheses. Or unused method arguments (it has some linting ability as well).

I didn't dig through all of them yet, but it seems reasonable. For instance, I'm no big fan of comments but a top level module comment per file might be nice for newcomers in a newcomer friendly project like ours.

What do you think?