update init step in brew installation instructions

This fixes issue: https://github.com/yyuu/pyenv-virtualenv/issues/36

Apply code blocks style

https://www.python.org/dev/peps/pep-0370/

Manage Anaconda/Miniconda environment as a pyenv version

an executable is not found in it, automatically look up the executable
for source Python version. (fixes #62)

init: fix shell detection when invoked from a script

- fixes `ps` command to lookup parent process
 - adds `assert_output_contains_not` test helper

Ref: https://github.com/yyuu/pyenv/issues/373

It will be done by `get-pip.py` and we don't have to do anything explicitly.

First release after my son's birth....

activate: do not display note if initialized already

Use `typeset -g` with `precmd_functions`

This fixes displaying the note, in case "init" has been run manually
and/or the init is not in the expected place.

It removes the unnecessary calls to "grep".

pyenv-virtualenv-init will look at `$PYENV_SHELL` now, too.

Tests have been adjusted/fixed.

Usage: version is optional

minor: fix comments

Link to English venv documentation

Currently the link to venv points to the Japanese version of the Python documentation.
Change this link to the English version of the Python documentation.

Ignore failure of pyenv activate

When the current pyenv environment is not a virtualenv, this line
exits with a return code of 1. This is annoying for those of us
who report the last return code in our own PROMPT_COMMAND, so
ignore the output of `pyenv activate` if we're not currently using
a virtualenv.

This makes it possible to use `eval "$(pyenv virtualenv-init -)"` from a
function.

fix wrong output in test in fish, refs 240a1de

"(any command)" is not supported in test in fish, we have to set a
variable and then use that

fix warns for virtualenv-init corner case on fish

The situation is when in a directory containing python binary,
`pyenv prefix' will return empty (caused by `pyenv which' returning
"./python"). This change adds quotes for `test', otherwise `test' will
warn about missing argument.