[Seq] Code refactor and bug solving

Here is a list of what I did:

* Split the code in several files (#28)
* Changed many function definitions to one-liners
* `nucleotide_count` was changed to the contructor `NucleotideCounts`
* Implemented (badly) a `NucleotideCounts` method for K-mers
* Solved  bugs (including ones with constructors, string decorators)
* Simplified nucleotide's `show()`
* Changed print formatting of sequences
* Improved commenting