Convert YamlNode to AbstractYamlNode

[SherifWaly]

PR for #91

I found that there are YamlMapping and AbstractYamlMapping, same for sequence.
So I removed YamlMapping and merged it with AbstractYamlMapping.

[coveralls]

Coverage increased (+0.02%) to 86.529% when pulling e5e5601 on SherifWaly:#91 into 142ef59 on decorators-squad:master.

[SherifWaly]

@amihaiemil Please check whether these changes are good or not. The problem came from the abstract classes as in PR description. Also I think some docs should be modified.

[amihaiemil]

@SherifWaly The AbstractYamlNode looks good, but you have to revert the removal of YamlMapping and YamlSequence, because:

• YamlMapping and YamlSequence are the interfaces that the user will work with -- see the repo's README.md for examples of how the user will work with the library.

• On the other hand AbstractYamlMapping and AbstractYamlSequence are supposed to be super-classes for any concrete implementation of YamlMapping and YamlSequence: they provide generic methods such as equals, hashcode and compareTo which are supposed to work with any implementations of mapping and sequence.

Notice that the 2 interfaces are public, the user will work with them, while the abstract classes are not public, the user will never know they exist (they are what's generically called "details of implementation")

You might not see the big picture entirely because I did a lot of the architecture by myself -- I'll write an architecture.md file, where I'll describe everything in detail.

In the meantime (and I wanted to tell you this before, too), don't hesitate to open issues about anything that seems unclear to you :D

[SherifWaly]

@amihaiemil Thanks for reviewing, I think YamlMapping and YamlSequence must be YamlNode so how to convert them to AbstractYamlNode and at the same time they are interfaces not classes?

[amihaiemil]

@SherifWaly Yes indeed.. and there's also that Yaml dump part... I guess it's a little more complicated than I initially thought. I'll look into it as well and let you know. For now, don't do anything else :D

[amihaiemil]

@SherifWaly I think it's better to leave this for now. It's quite a big change, as it seems, and I want to release the first version soon :)

[amihaiemil]

@SherifWaly I'm closing this PR - the issue is postponed, and by the time we'll take care of it, this PR will long be outdated :D