Organizational updates. Mega2560 support moved.

- Mega2560 support has been moved to the Grbl-Mega
[project](http://github.com/gnea/grbl-Mega/) to clean up the code and
make future developments easier with increased flash and RAM. All new
developments between the 328p and Mega2560 will be synced when it makes
sense to.

- OEM single file compile configuration option. Before OEMs needed to
alter three files. Provided a way to just alter the config.h file to
contain everything for a particular Grbl build. See config.h for more
details.

- Removed defaults and cpu_map directories and reverted back to
defaults.h and cpu_map.h to contain all definitions. This should help
reduce some headaches and the previous implementation inadvertently
created. Also, it makes the single file config.h possible.

- Moved (and tweaked) the invert control pin mask define and placed
into config.h, rather than in the cpu_map.h file. Makes more sense
there.

doc/log/commit_log_v1.0c.txt

@@ -1,3 +1,14 @@
+----------------
+Date: 2016-03-19
+Author: Sonny Jeon
+Subject: No variable spindle and spindle speed fix.
+
+- Soft limit errors were stuck in a feed hold without notifying the
+user why it was in a hold. When resumed, the soft limit error would
+kick in. Issue should be fixed to behave as intended to automatically
+hold and issue a soft limit alarm once the machine has come to a stop.
+
+
 ----------------
 Date: 2016-03-11
 Author: Sonny Jeon