Clearly identifying memory and execution time limits during install

We've seen many errors coming out of our past attempts at fixing memory issues during install. To rectify this issue, we're adding a requirements page on commons so anyone with a memory limit under 128m and max execution time under 120 will get an error when installing.

This depends on the following core patch:
#1971072: Install profiles cannot list their own requirements in drupal_check_profile

Once this patch is in, we will change the installer so it does the following:
a) throw an error if memory is under 128m
b) up the installer memory to 196m if setting is between 128m and 196m
c) do nothing if setting is above 196m

For max_execution_time
a) throw an error if max_execution_time is under 120
b) up the installer max_execution_time to 250 if setting is between 120 and 250
c) do nothing if setting is above 250

This should fix problems for people who have slow processors (and execution takes longer) and those that want efficient memory usage (128) but don't want the installer to fail.

Anyone not meeting the minimum requirements should get an error when trying to install, and we cannot support setups that have less than 128m or a 120 second max_execution_time