add nb.test() by wrapping nose.main()

unlike Numpy which create a NoseTester class to handle testing,
currently I just wrap nose.main() and stop testing if no nose exists.

So it tries to load nose every time we load the nextbiopy, which may not
be a good idea. I think some refinement is desired.
Revert "add nb.test() by wrapping nose.main()"

This reverts commit 1bf85138542ef2c4637df66a038928174d08b8f2.
Finish basic structure class Seq

Refer to issue #6, three attributes are respectively created. Exception
will be raised if the length of seq and qual mismatch.

Also the correspondent unit tests and doc are added.