rbenv exec: avoid mutating PATH

Enables shelling out from a ruby process started with rbenv to a ruby
process with a different RBENV_VERSION. Fixes #121

This removes the workaround created for #15 and solves `ruby -S` support
by setting RUBYPATH. PATH is never changed.

To illustrate how RUBYPATH changes in various configurations:

    PATH=~/bin:~/.rbenv/shims:/usr/bin:/bin
    RBENV_VERSION=1.8 ruby -S rake
    #=> executes ~/.rbenv/versions/1.8/bin/rake
    #=> RUBYPATH=~/bin:~/.rbenv/versions/1.8/bin:/usr/bin:/bin

    RBENV_VERSION=2.0 ruby -S rake
    #=> executes ~/.rbenv/versions/2.0/bin/rake
    #=> RUBYPATH=~/bin:~/.rbenv/versions/2.0/bin:/usr/bin:/bin

    RBENV_VERSION=system ruby -S rake
    #=> executes /usr/bin/rake
    #=> RUBYPATH=~/bin:/rbenv_shims_were_here:/usr/bin:/bin

    RBENV_VERSION=1.8 ruby -S rake
    #=> executes ~/.rbenv/versions/1.8/bin/rake
    #=> RUBYPATH=~/bin:~/.rbenv/versions/1.8/bin:/usr/bin:/bin
Install a more recent version of Ruby in README instructions
check if completion script is readable

fixes #444
fix rehash when paths have spaces in them

fixes #450
add fish shell support
cleanup in fish

Use process subtitution syntax:

    . (rbenv init -|psub)

instead of:

    eval (rbenv init -)

because the latter doesn't work well with newlines.
fix `rbenv()` function in ksh and dash

ksh syntax becomes:

    function rbenv {
      typeset command

`typeset` only declares a local variable if there's an explicit
`function` declaration; otherwise the variable leaks.

Other shells use this syntax:

    rbenv() {
      local command

This is for dash compatibility, which supports neither `function` nor
`typeset`.

references #205, fixes #408
Merge branch 'shell-fixes'
Ubuntu fix: don't use `.` in place of `source`

If `rbenv init -` outputs `.` in place of `source` and that gets eval'd
by the desktop manager via `~/.profile`, it chokes and prevents the user
from logging in.

Fixes #457
better Ubuntu Desktop installation instructions

Don't suggest that they add to their `.profile` anymore because:

1. They would have to restart the desktop session for their change to be
   reflected, or source the file manually;
2. An error in `.profile` may prevent logging in;
3. The `rbenv()` shell function and shell completions won't be available
   in Terminal since it doesn't start bash in login mode by default.

Therefore, suggest that they use `.bashrc` instead. This will be
immediately reflected in a new Terminal tab. If bash is started in login
mode somehow, the default `.profile` is set up to source `.bashrc` anyway.

Also, don't suggest restarting the shell environment with
`exec $SHELL -l`, since we don't know what was the original mode that
their shell was started in. (OS X Terminal.app will be login mode,
Ubuntu Terminal has non-login mode by default.) Mode can be checked with:

* bash: `shopt -q login_shell`
* zsh: `[[ $options[login] = "on" ]]`

But since this is gnarly, let's just avoid it altogether and go the
easy route.

Closes #305, fixes #373, reverts #286
remove `rbenv install` example; clarify manual compile

It can be confusing if people mistake the example as instructions to
install ruby-build with.
tweak installation instructions

Provide an `rbenv install` example and explanation lower down, so that
both folk that are following git install and Homebrew install will
finish at the same section and learn how to use ruby-build.

Also remove step 6 of manual install process: `rbenv rehash`. It's
unnecessary at that point, even if they already did `rbenv install`
(which rehashes automatically).

Fixes #455, closes #456
better error message for `rbenv prefix system`

Have `rbenv prefix` handle the case where system Ruby is not installed,
i.e. `rbenv which ruby` doesn't find ruby in PATH.

Fixes #362
typo: remove "v" (version prefix)
reliably detect parent shell in `rbenv init`

`$SHELL` variable is a terrible way of detecting the current shell
because it's not even supposed to reflect the current shell; it's meant
for keeping the value of the default shell for programs to start.

If an explicit `<shell>` argument wasn't passed to `rbenv init`, it
tries to detect the shell by getting the name of its parent process. If
this fails, it falls back on the value of `$SHELL` as before.

Furthermore, `rbenv init` will set the RBENV_SHELL variable in the
current shell to the value of the detected shell so that `sh-shell` and
`sh-rehash` commands don't have to repeat the detection.
strip initial `-` from the name of shell process

Avoids the case where shell detection would result in
`basename "-bash"` invocation, triggering the error:

    basename: illegal option -- b

fixes #462
Add more tests for version-file-read
Simplify version-file-read to avoid process substitution

Process substitution seems to be causing problems with some versions of
bash on specific systems.

Fixes #401
Merge branch 'no-process-substitution'
Merge pull request #372 from sstephenson/exec-next

Change `exec` semantics to allow using rbenv from a ruby process