Fix switch-phpfarm.

test -e does not work on a symlink whose target does not exist (1 is returned).
Since the idea is to replace the link, we only need to force the action.
Fix removal of obsolete versions.

The compilation directory and all its files were successfully removed,
but the installed files were left untouched.
Also improves prompt format when asking whether things should be removed.
Use custom process to install pear.

This gives us more control about the way pear gets installed and also
allows us to create a pear installation structure that matches the one
Pyrus uses.
Minor changes.
[BC] Change flags order and add flag for pear.

The flags no appear in alphabetical order in the version string.
A new "pear" flag has been added to indicate that a PEAR installation
is required.
[WIP] Change the compilation phase.

The idea is to allow multiple SAPIs to be compiled and installed
(namely, the CGI and FPM SAPIs).

This patch also disabled the automatic installation of PEAR since there
is a new flag ("pear") to decide that now.
Update README.