Some major formatting, code-style and other fixes

This is the first part. Please merge it into your master before continuing with work.

## Add .editorconfig

EditorConfig helps developers define and maintain consistent coding styles between different editors and IDEs. — http://editorconfig.org

As you can see in [.editorconfig](https://github.com/jirutka/asciidoctor-latex/blob/patch-2/.editorconfig), it instruct editor/IDE to:

1. use UTF-8,
1. use `\n` (LF) for line endings (i.e. unix standard),
1. ensure that files ends with a newline (generally highly recommended for text files),
1. remove any whitespace characters preceding newline characters, i.e. trailing whitespaces (to not mess commits with redundant whitespace changes),
1. use two spaces for indentation, not tabs in ruby sources (to be consistent and sane).

I highly recommend to install [EditorConfig plugin](http://editorconfig.org/#download) into your editor or IDE, so it’ll automatically take care of these basic code-style rules (i.e. it automatically removes trailing whitespace when you save file etc.)

## Clean trailing whitespaces and blank lines, insert final newline

Fixes violations of the rules mentioned above and in addition removes unnecessary multiple blank lines at the end of file.

## Fix mismatched indentation in ruby sources

There were mixed spaces and tabs for indentation (please never ever do this). The Ruby style conventions recommends using two spaces for indentation.

## Fix some formatting

Some minor formatting and code-style improvements.

## Fix Gemfile and gemspec

It’s unnecessary to repeat the same dependencies both in gemspec and Gemfile, just put `gemspec` into Gemfile.