Fix build on MSVC

Fix build on MSVC

* Download all snapshots and such from static.rust-lang.org (drive-by fix)
* Update dependencies to work and build on MSVC
  * ssh2-sys uses `nmake` to build libssh2
  * libgit2-sys uses `cmake` to build libgit2
  * curl-sys uses `nmake` to build libcurl
  * libz-sys uses `nmake` to build zlib
  * miniz-sys uses `gcc-rs` to drive `cl.exe` manually
  * term updated to pick up windows-specific deps on MSVC
* Updated .travis.install.deps.sh to install MSVC target libraries on Windows,
  so the compiler can be used to target MSVC. A 64-bit target is now used by
  default as well.
* Updated the configure script and Makefile to set appropriate environment
  variables to build Cargo from a MSYS shell with a MSVC target. This is similar
  to how Rust bootstraps on MSVC as well.

With this commit Cargo can successfully bootstrap itself on MSVC (use the MSVC
product to build itself again). The tests currently do not pass because
unwinding is not implemented, but soon!