Improve the output of `hit fetch`

Previously:

    $ hit fetch http://llvm.org/releases/3.4/libcxx-3.4.src.tar.gz
    Downloading http://llvm.org/releases/3.4/libcxx-3.4.src.tar.gz...
    Downloading 'http://llvm.org/releases/3.4/libcxx-3.4.src.tar.gz'
    [=========================] 100.0% (1.3MB of 1.3MB) 2.418MB/s ETA 0s

    tar.gz:xwpsaatz6dvrt7bi5uwz2yd57u4ofz6z

Now:

    $ hit fetch http://llvm.org/releases/3.4/libcxx-3.4.src.tar.gz
    Downloading http://llvm.org/releases/3.4/libcxx-3.4.src.tar.gz...
    Downloading 'http://llvm.org/releases/3.4/libcxx-3.4.src.tar.gz'
    [=========================] 100.0% (1.3MB of 1.3MB) 3.022MB/s ETA 0s

    url: http://llvm.org/releases/3.4/libcxx-3.4.src.tar.gz
    key: tar.gz:xwpsaatz6dvrt7bi5uwz2yd57u4ofz6z

The output can now be copy & pasted into the package.yaml
Improve the output even further

Actually, it is even better to simply output the actual lines that you can copy
and paste, so now the output is:

    $ hit fetch http://llvm.org/releases/3.4/libcxx-3.4.src.tar.gz
    Downloading http://llvm.org/releases/3.4/libcxx-3.4.src.tar.gz...
    Downloading 'http://llvm.org/releases/3.4/libcxx-3.4.src.tar.gz'
    [=========================] 100.0% (1.3MB of 1.3MB) 3.523MB/s ETA 0s

    sources:
    - url: http://llvm.org/releases/3.4/libcxx-3.4.src.tar.gz
      key: tar.gz:xwpsaatz6dvrt7bi5uwz2yd57u4ofz6z
Merge pull request #239 from hashdist/fetch_output

Improve the output of `hit fetch`