Correct handling of unicode escapes within double quotes.

The problem with unpacking a str node was that unicode escape sequences
are translated to unicode code points in the text value. We must look
at the original source code, which we can get from the node too.
New cop `EndAlignment`
Merge pull request #248 from jonas054/fix_double_quoted_unicode

Correct handling of unicode escapes within double quotes.
Add progress formatter
Fix severity name typos
Rename PlainTextFormatter to SimpleTextFormatter
Change Eval's severity to warning
Validate severity type of Offence
Merge pull request #249 from yujinakayama/progress-formatter

Add progress formatter