Add notes for using Eclipse to build and debug.

Thanks to David Cabanis for sending me the notes that showed me how to get
this working.  These notes are heavily influenced by his work.
Make sure heap not used from ISRs.

Wrapped malloc/realloc/free calls to make sure that they aren't being
called from within an ISR since this will typically lead to heap
corruption as the heap code isn't re-entrant.
Rebuild sources if makefile changes.

Now if you touch the settings in your main makefile, the code will be
rebuilt without you needing to force a clean build manually.
Provide _sbrk() which checks for overflow.

Now if you try to make so many dynamic memory allocations from the heap
that the top of the heap would exceed the stack pointer, it fails to do
the allocation.

NOTE: Still possible for the stack to grow down into the heap and cause
      corruption but this does reduce the chances of that happening.