Add possibility to use inherit_from in .rubocop.yml.

This adds functionality for inherit_from and uses it in the new
config/*.yml files, and also from our own .rubocop.yml.

Default when no .rubocop.yml is found is to use config/default.yml,
and all configuration files inherit implicitly from config/default.yml.

Default values are not handled in code anymore, since full
configuration is always present.
Merge pull request #143 from jonas054/based_on

Add possibility to use inherit_from in .rubocop.yml.
Merge pull request #145 from zeroed/master

Fix #136
Handle method names with a capital initial.

This is legal ruby even through the method name is a constant
rather than a normal identifier. Fixes #144.
Merge pull request #146 from jonas054/fix_single_line_methods_bug

Handle method names with a capital initial.
Display a list of errors in the summary.

If there are errors when running rubocop on a large code base, it can
be difficult to find where errors occurred if you only see the number
of errors in the summary.

Also, red is a better color for these messages than yellow.
Merge pull request #147 from jonas054/easier_debugging

Display a list of errors in the summary.
Mention mailing list in the README
Handle regexp back references in VariableInterpolation.

This is a bug fix. Strings like "#$1" caused a crash earlier.
Merge pull request #152 from jonas054/fix_errors

Handle regexp back references in VariableInterpolation.
Remove extra / in target file pathnames

When you passed a directory like some/ to RuboCop you got pathnames like
~/projects/some//test.rb because an extra / was added no matter whether
it was needed or not.
Update changelog
Do more caching to avoid printing same warning many times.

With the addition of support for `inherit_from`, the configuration file
loading became more complicated. A side effect was that the warning
about unrecognized cop would be printed many time per `.rubycop.yml` file.

I've changed the config caching machanism so that one cache holds
config file paths and another holds config objects. The API of the
`Config` class was changed and big changes in specs were necessary.

Fixes #151.
Simplify spec a bit
Merge pull request #153 from jonas054/print_unrecognized_cop_once

Do more caching to avoid printing same warning many times.
Bump version to 0.7.0
Fix #154 - Add missing files to gemspec
Bump version to 0.7.1
Fix for #155. A bug in Semicolon#index_of_first_token_on_line.
Merge pull request #156 from jonas054/semicolon_bug

Fix for #155. A bug in Semicolon#index_of_first_token_on_line.