Compile libsodium statically

Title says it all. This increases the size of the binaries a bit (~11MB, up from 9.1MB), but now users don't have to have libsodium installed! Of course, libsodium is still required for building Sia, so developers will still need it.

A new target has been added to the Makefile for creating releases. This target compiles libsodium statically and creates an archive from the resulting binaries.

A companion pull request to gh-pages, reflecting the new installation instructions, will be made once this change makes it into the release branch.