add check-commits target

This adds a 'check-commits' target to the build system. This is very useful for checking patch series, it will run through each invidividual commit on top of origin/master and execute the tests at each step.

The process can be customized through environment variables;

CHECK_BASE - the base object. defaults to 'origin/master'
CHECK_HEAD - the head object. defaults to 'HEAD'
CHECK_TARGET - the list of targets to build. defaults to 'all;test'. can be set to 'all' to just perform build tests.
CHECK_THREADS - number of threads to use when building (make -jX). defaults to number of logical cores on the system. might need some adjustments on OSX.

I always put this in all my build systems. And really miss it when i work on OPM. Hopefully we can scratch that itch now =)

Cosmetics, et al; i did not put much effort into those. just point out what you don't like, i'll gladly adjust.
I get
```
nproc: command not found
```
when I attempt to use it on CentOS 5.11, which means `build_threads` is empty.  Maybe we could have something like
```cmake
if (UNIX)
  execute_process (...)
  string (REGEX REPLACE ...)
endif ()

if (not build_threads)
  set (build_threads 1)
endif ()
```