Change keyboard selection mechanism

Remove vertical wraparound.
This closes #8.
Refactor MSAFile.

- Add unique_rows property.
- Remove unique_count property. Use unique_rows.length instead.

MSAFile forms no longer a tree and as such is not easily
serializable.
Refactor MSARow.

For non-unique rows reference the original alignment
directly.
Keep comments from the original, when saving.

In fact every line, which is not recognized as an alignment
is preserved.
This closes #6.