New pydna release 3.1.0

[Björn Johansson]

Hi all and sorry for the delay in getting a new version out.

Most important fix is removing references to the biopython Alphabet

that has been removed from biopython.

There are changes to how primer melting temperature is calculated.

I had a hard time to figure out what is the state of the art in this regard.

Look at comments in pydna.tm.

Also, the project now has the src layout so that it is easier to run tests on

installed code.

As always, I am open for suggestions. 

1. I am thinking of adding an improved gel simulation module to replace the one in pydna. 
2. More cloning procedures like golden gate ?

Let me know your thoughts here in the google group so that as many people as possible can participate.