Optimize pyenv-which: implement remove_from_path in Bash

This greatly improves the performance of `pyenv virtualenvwrapper_lazy`,
which happens to call pyenv-which a lot.

For profiling I've initially used:

    % zmodload zsh/zprof
    % eval "$(pyenv init -)"
    % pyenv virtualenvwrapper_lazy

    Before:
    % zprof|grep -E '(pyenv|virtualenv)'
     1)    1         754,07   754,07   58,95%    751,50   751,50   58,75%  pyenv
    21)    1           2,57     2,57    0,20%      2,57     2,57    0,20%  virtualenvwrapper_setup_lazy_loader
     1)    1         754,07   754,07   58,95%    751,50   751,50   58,75%  pyenv
           1/1         2,57     2,57    0,20%      2,57     2,57             virtualenvwrapper_setup_lazy_loader [21]
           1/1         2,57     2,57    0,20%      2,57     2,57             pyenv [1]
    21)    1           2,57     2,57    0,20%      2,57     2,57    0,20%  virtualenvwrapper_setup_lazy_loader

    After:
    % zprof|grep -E '(pyenv|virtualenv)'
     1)    1         383,30   383,30   27,97%    380,88   380,88   27,79%  pyenv
    31)    1           2,42     2,42    0,18%      2,42     2,42    0,18%  virtualenvwrapper_setup_lazy_loader
     1)    1         383,30   383,30   27,97%    380,88   380,88   27,79%  pyenv
           1/1         2,42     2,42    0,18%      2,42     2,42             virtualenvwrapper_setup_lazy_loader [31]
           1/1         2,42     2,42    0,18%      2,42     2,42             pyenv [1]
    31)    1           2,42     2,42    0,18%      2,42     2,42    0,18%  virtualenvwrapper_setup_lazy_loader

Fixes https://github.com/yyuu/pyenv-virtualenvwrapper/issues/13
v20140628
Remove `exit 0` from initialization code of fish
Add CPython 2.7.8 (refs #201)
Support `SETUPTOOLS_VERSION` and `PIP_VERSION` to allow installing specific version of setuptools/pip (#202)
v20140705 (fixes #204)
Display the package name of `hg` as `mercurial` in message (#212)
Fix zsh completion with multiple words

`${words[2,-2]}` must not be quoted, otherwise completion for multiple
words (e.g. `shell 3.4.1`) fails.
Unset `PIP_REQUIRE_VENV` during build (fixes #216)
Merge pull request #215 from blueyed/fix-zsh-completion

Fix zsh completion with multiple words
Set MACOSX_DEPLOYMENT_TARGET from the product version of OS X (#219, #220)
Fix broken tests
Add tests for MACOSX_DEPLOYMENT_TARGET (#219, #220)
Setup configure options to build against universal SDK on Darwin
Merge pull request #222 from yyuu/macosx-deployment-target

Setting MACOSX_DEPLOYMENT_TARGET on Darwin
Add Jython 2.7 Beta 3 (fixes #223)

This release of Jython requires JDK 7 or above
v20140825
doc: note about using multiple versions

Add a note about how to activate/use multiple versions to the main
README, to the "Choosing the Python Version" section.
minor: remove trailing whitespace
Merge pull request #227 from blueyed/doc-multiple-versions

doc: note about using multiple versions