Using php5enmod if wanted (fixes #107)

Hi @djoos,

I decided to go for a config option because:
- It's difficult (impossible?) to know which PHP version will be installed. For instance if the recipe is included during the initial run list (our case), PHP won't even be yet installed when recipe converges
- Some folks (not me) may want unusual configs and should still be able to specify another directory whatever their PHP is. (otherwise the config_file option makes no sense)

So I:
 1) Added the said config
 2) Simply explain in the Readme what should be the default config
 3) Added some basic unit tests on this (in particular included https://github.com/escapestudios-cookbooks/newrelic/commit/93f0cce92bbf27458eda23ae2b77b7d4944cd1f7 from https://github.com/escapestudios-cookbooks/newrelic/pull/117 otherwise recipe does not converge)

Note: Given http://www.lornajane.net/posts/2012/managing-php-5-4-extensions-on-ubuntu it's actually needed for php >= 5.4