Amqp extension

[stof]

This implements #116

While working on it, I understood the issue faced in #122: the buildpack was not supporting extensions which depend on an external library. Given that AMQP does, I added such support. This will allow to fix the memcached compilation once this is merged.

[stof]

OK, I'm merging this one to be able to continue working on other extensions. If there is some feedback provided after the merge, it will be handled later.

[stof]

The extension is now available for PHP 5.5 (others are coming)

[phansys]

Hi @stof.
I have the same issue with this buildpack, but with https://github.com/heroku/heroku-buildpack-php everything is fine.
I need this buildpack to execute post install tasks such as assetic:dump
This is my Heroku output:

-----> Vendoring Composer
       Updating Composer
-----> Bundling additional extensions gd
intl
mbstring
memcached
       gd
       intl
       mbstring
       memcached
Updating to version 4ecdbf89c4a3d1e5dfe73c57e3202a5e2a18c87e.
    Downloading: 100%         
-----> Installing application dependencies with Composer
       Loading composer repositories with package information
       Installing dependencies from lock file
       Your requirements could not be resolved to an installable set of packages.

         Problem 1
           - The requested PHP extension ext-memcached * is missing from your system.


 !     Push rejected, failed to compile Multipack app

Any clue?
Thank you in advance.

[stof]

which PHP version are you using ?

[phansys]

My dependencies have a requirement for "php": "5.5.*".
Currently it is resolved by Heroku in this way:

-----> Bundling PHP 5.5.10