Fix encoding problem

Hi

We encounter some encoding exception when we try to generate `.fdoc` file and convert it into `.html`.

Therefore, I add the following changes:

* Add magic encoding (`# -*- coding: UTF-8 -*-`) to every ruby source file.
* Add UTF-8 specifying when dump yaml (`File.open(@endpoint_path, "w:UTF-8")`)

The above two changes fix the conversions error `Encoding::UndefinedConversionError: "\xE4" from ASCII-8BIT to UTF-8`. For more detail, please ref this [SO answer](http://stackoverflow.com/questions/1989348/ruby-1-9-yaml-and-string-encodings-how-to-lead-a-life-of-sanity)

In addition, I replace `JSON` by [Oj](https://github.com/ohler55/oj) to fix the conversion error which occurs when I try to convert `.fdoc` into `.html`. Moreover, this gem is faster than the standard JSON library. Please also ref this [SO answer](http://stackoverflow.com/questions/13003287/encodingundefinedconversionerror).

This pull request is related to #27 and should fix that problem.

Thanks