Make a "release" directory in the sagenb repo that generates a new proper sagenb spkg

[was]

The top level of the sagenb spkg does not have an hg repository, which is very confusing since all spkg's are supposed to. It turns out that spkg-install is autogenerated and SPKG.txt is in sagenb's own repository, i.e., in sagenb-0.9.0/src/sagenb/SPKG.txt.

Jason Grout and William Stein discussed this for a while and propose the following plan:

1. Create a directory sagenb-0.9.0/src/sagenb/release/sage
2. Put spkg-dist, SPKG.txt there, along with a small hg repo that contains SPKG.txt and spkg-install.
3. Use this new spkg-dist script to generate a proper sagenb spkg in the usual way. However, it will copy over the hg repo created in 2, then check in spkg-install and SPKG.txt.

The motivation for this is so that the sagenb spkg is the same as the other ones, which could be important for automated tools that make a directory of spkg's, etc.