Add PMBus class

This class provides the interfaces to read PMBus devices
via sysfs files.

This commit provides 3 interfaces:
 1) Write an integer (to provide: echo 1 > clear_faults)
 2) Read a file that represents a bit in a register.
 3) Read a file that represents a bit in a register within
    a PMBus page (or in something that acts like a page).

Additional ones may be added in the future.

ReadFailure/WriteFailure exceptions will be thrown on errors.

Change-Id: I7ea166da00ddc558a9f25c07e6ad9a16714cc3b6
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/Makefile.am b/Makefile.am
index 72698b7..20ab498 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -10,6 +10,7 @@
 	$(SDBUSPLUS_CFLAGS)
 
 libpower_la_SOURCES = \
+	pmbus.cpp \
 	timer.cpp \
 	utility.cpp