Add code style formatting (pretty) and checker (pretty-check) targets. (#52)

* Add 'pretty' target to reformat sources to be compliant with code style.
* Add 'pretty-check' target to check if sources are compliant with code style.
* Update travis-ci script to use 'pretty-check' and enforce code style.
* The 'pretty' and 'pretty-check' targets rely on the 'astyle' tool.

tests/Makefile.am

@@ -42,6 +42,12 @@ SUBDIRS                                 = \
     unit                                  \
     $(NULL)
 
+# Always pretty (e.g. for 'make pretty') these subdirectories.
+
+PRETTY_SUBDIRS                          = \
+    unit                                  \
+    $(NULL)
+
 if OPENTHREAD_BUILD_TESTS
 if OPENTHREAD_BUILD_COVERAGE
 CLEANFILES                             = $(wildcard *.gcda *.gcno)