Adding support for New Relic

This request adds support for the New Relic PHP agent to be automatically installed into a pushed app. This allows folks to use an existing New Relic account with their CF hosted apps.  The agent is downloaded, and cached if a NR_INSTALL_KEY environment variable is specified with the license key for that account, the php.ini file is modified with the appropriate settings for New Relic, and the newrelic.so and daemon are installed in the staging dirs.

If the user specifies their own php.ini file, then they would need to at least specify the environment variable with some value so that the agent binaries would be installed.