Accept multiple PHP_VERSIONS directories, version X.Y notation and add the "sbin" path to $PATH

Hi,

This PR adds three features:
- "sbin" path preprended in $PATH
- multiple PHP_VERSIONS directories
- X.Y version notation in addition to the classical X.Y.Z

**Why sbin?**
Because the php-fpm binary is not located under the bin subdir.

**Why multiple PHP_VERSIONS directories?**
Because sometimes you have your different PHP versions installed in different directories.
For example, if you install PHP 5.3.21 and PHP 5.4.11 with homebrew (and you don't want to mess your FS with symlinks), you will have the below structure:
- /usr/local/Cellar/php53/5.3.21
- /usr/local/Cellar/php54/5.4.11

You can even image, want to have 5.3.21 and 5.4.10 installed with homebrew, and compile your own PHP version with debug symbols.
Looking for the right PHP version in multiple directories allow you that without having to simlink everything in the right directory.

**Why the X.Y notation?**
Because, as specified in the documentation, to activate a default PHP version, you have to put the specific version.
However, when you activate a version, often what your are interested in is the major PHP version.
This also avoid to use the autocomplete when you just want a major PHP version.

I don't know if your are interested in these features as you want to keep php-version simple, so let me known if you want only a subset.
Also, if your prefer three PR - one for each feat. - I can split this one.