Add encoding support

[ForNeVeR]

For effective implementation of atom/atom#1806 we will need alternate encoding support in text-buffer project.

IMO it should be able to get encoding from the constructor params, read file with this encoding and remember it so it can save file with the same encoding it was read with. Also it should be optional encoding parameter for the save function so caller may override the encoding on save.

We could also add automatic encoding detection feature in future.