Add stack sdist

[mgsloan]

stack upload now no longer depends on the cabal binary, as #313 is implemented. However, this doesn't write out a tar file, and it's still desirable to be able to write a tarball to the disk. It should be trivial to add a stack sdist command to do this.

It would also be really cool to have something like stack sdist --test. This would create the sdist tarball, unpack it, build it, and run the tests.

[mgsloan]

#654
f5f5108

[mgsloan]

Done!