Add K9LogPatternParseResult

[K9LogPatternParser parse:error:] now returns K9LogPatternParseResult
instance instead of plain NSArray.

- Move message construction code in [K9LogLumberjackPatternFormatter
  formatLogMessage:] to [K9LogPatternParseResult stringFromLogMessage:]
- K9LogPatternParseResult simply wraps NSArray
- It supports NSFastEnumeration
- It also supports LLVM 4.0 "Indexed Object Subscripting"
  - See http://nshipster.com/object-subscripting/ for more details
Remove TODO comment

At first I supposed that providing abstract class for initialization
boilerplate code is nice idea. I've realized, however, keeping
K9LogLumberjackPatternFormatter simple NSObject subclass is more
important.