api: unexport setEnv handler

Related to #289.
api: unexport unsetEnv handler

Related to #289.
api: unexport getEnv handler

Related to #289.
docs/overview: marking this doc as alpha and linking github issue for suggestions
docs/apps: fixing typo in post restart hook name
api: remove named result usage
app/bind: display only the value of the variable in env.String
api: refactoring getEnv handler

Now the handler receives a JSON, and returns another JSON. I will
document it better in the future API documentation, but this is how it
works:

  * receives a list of strings (the name of the apps) or nothing (an
  empty string or an empty JSON list).
  Example: ["DATABASE_HOST", "DATABASE_USER"]
  * returns a JSON object, where environment variable names are
  attributes, and env values are attribute values.
  Example: {"DATABASE_HOST": "localhost", "DATABASE_USER": "root"}
provision/local: add workaroud for the unreachable code issue

This code works uniformly in Go 1.0 and Go 1.1.
api: refactoring setEnv handler

Now the handler receives the multiple environment variables in the
format of a JSON object, where attributes represent name of variables
and values represent values of variables.

It still exports only public environment variables, this change affects
only the format of data, not the behavior of the handler.
api: refactoring unsetEnv handler

Now the handler receives a list of strings in JSON format, each string
represent the name of a environment variable. It's a change in the data
format, nothing changed in the behavior of the handler.
cmd/tsuru-base: refactor env-get and env-unset commands

Also testing the previously untested request body. env-set still needs
love :-)

Related to #377.
cmd/tsuru-base: fix env-set behavior

Now it uses JSON for communication with the server. I've also included a
test reproducing #377 and fixed it.

Fix #377.
cmd/tsuru-base: don't let request body open on requestEnvUrl
cmd/tsuru-base: fix env-get

Now it handles JSON response properly.
cmd/tsuru-base: display environment variables in alphabetical order
cmd/tsuru: version 0.7.2
api: requiring tsuru 0.7.2

The server has just changed how it handle environment variables. All
environment variables handlers now use JSON as the data format in the
wire.
cmd/tsuru-base: print new line character after printing env vars
cmd/tsuru: version 0.7.3