[#1220] Create namespace Metrics and move some cops there

Move the Style cops BlockNesting, ClassLength,
CyclomaticComplexity, MetricsLineLength, MethodLength,
and ParameterLists to the new namespace.
[Fix #1220] New cop PerceivedComplexity

Configurable Max and with special support for --auto-gen-config.

Counts else nodes but does not count when nodes.
[Refactor] Extract MethodComplexity mixin module
Give warning when a cop name has the wrong namespace

This is more or less necessary when we move cops from one
namespace to another. If there are no ambiguities, we
automatically change to the right namespace at run-time,
not changing any files.
[Refactor] Make Cop class smaller
Merge pull request #1231 from jonas054/1220_new_complexity

New cop Metrics/PerceivedComplexity