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
pyenv-version-file-write: do not use rm, but clobber the file

With e.g. /usr/local/bin/.python-version owned by some user, `pyenv
local foo` would fail, if the user has no permissions for
`/usr/local/bin`, but only the `.python-version` file.
Merge pull request #230 from blueyed/version-file-write-no-rm

pyenv-version-file-write: do not use rm, but clobber the file
Add PyPy 2.4-beta1 (fixes #232)