Detect RAM usage and stop if full

This resolves issue #1356 and add the ability for the Arduino IDE to
detect the amount of RAM allocated to a sketch and compare that to the
available RAM on each board. If RAM is more than 90% full, it will fail
on building since there is not enough free RAM for the heap and stack to
use.
Don't report memory usage if it's unknown
Clarified error messages and added a configurable warning level

Changed memory usage check to only fail build on 100%+ usage and added a
configurable warning level for memory usage defaulting to 75%.

Clarified error and warning messages related to memory usage to specify
that this is the minimum memory usage.
Merge branch 'penguin359-ide-1.5.x-ram' into ide-1.5.x

Fixes #1377
Fixes #1356