[Fix #1097] Add namespace for cop names in configuration

LineLength becomes Style/LineLength etc.
Merge pull request #1103 from jonas054/namespaces_in_config

Namespaces for cop names
Remove unnecessary hash merge in TargetFinder

The typo fail_level instead of fail_fast didn't affect function, so
that whole thing is unnecessary. Let values default to nil instead of
false. Works the same.
[Fix #1106] EachWithObject with method call body
Merge pull request #1108 from geniou/each_with_object_fix

[Fix #1106] EachWithObject with method call body
Merge pull request #1107 from jonas054/fix_fail_fast

Remove unnecessary hash merge in TargetFinder
Make exception for JRuby in setting yamler to syck

Avoids the warning
JRuby 1.9 mode only supports the `psych` YAML engine; ignoring `syck`
Merge pull request #1110 from jonas054/fix_jruby_psych_warning

Make exception for JRuby in setting yamler to syck
Add section about custom cops [ci skip]