Give install-time warning if insufficient memory or page execution time

Problem/Motivation

A major source of install and runtime issues with Open Outreach (as with most or all most-used Drupal 7 distros) is too-low values PHP values of either available memory (memory_limit) or page execution time (max_execution_time).

While we highlight appropriate values in documentation, many site installers will install without reading the documentation.

Proposed resolution

Implement hook_requirements() to set a warning if there is insufficient memory available or insufficient page execution time.

Follow relevant code in D8: drupal_check_memory_limit(), simpletest_requirements().