readme: describe restore and fix add/update

The procedure for adding and updating dependencies was
out of date. This commit splits them into separate
sections and corrects the steps to reflect reality.

It also adds a section describing the restore command.

closes #91
warn on -copy=false

As discussed in http://goo.gl/RpYs8e, we will be
removing the ability to save dependencies without
copying their source code. Warn users now of this
upcoming change.
warn while reading deps if -copy=false was used

As discussed in http://goo.gl/RpYs8e, we will be
removing the ability to read dependencies without
source code. Warn users now of this upcoming change.
better error message in test code

This makes debugging the test code easier. For example,
while writing a file tree for use by a test case, the
git comand might exit with a nonzero status. It's
helpful then to see what command we were trying to run.
fix migrating from -copy=false to true

Flag -copy is used to determine where to write out the
newly-generated manifest, but it was also incorrectly
being used to locate any existing manifest file. This
meant that when switching to or from -copy, reading the
existing manifest was broken. We fix this by attempting
to read an existing manifest in both locations,
regardless of the given flags.

We also explicitly check that the correct version of
each dependency package is installed in GOPATH, when
switching to -copy=true.
save: remove flag -copy=false

As discussed in http://goo.gl/RpYs8e, we are removing
the ability to save dependencies without copying their
source code.
update go subrepo to new import path

See https://groups.google.com/d/msg/golang-nuts/eD8dh3T9yyA/l5Ail-xfMiAJ.