Update mbed SDK sources to silence GCC warnings

When building the GNU tools for ARM Embedded toolchain, I use the -Wall -Wextra flags.  This was causing a few warnings to be emitted when building the mbed SDK sources.  I have gone through the code and eliminated these warnings when building it for LPC176X, LPC11UXX, and KL25Z.

Most of the changes were related to two issues:
* Enumerations (ie. ADCName, PinName, etc) would be compared to constants that had been cast to uint32_t instead of the correct enumeration type.  These enumerations can be smaller than 32-bit when compiled by GCC so a warning was thrown.  Were these cast to uint32_t to satisfy another compiler?
* GCC throws warnings when there is a mismatch between the signs of the values used within a comparison.  In most of these cases, I cast the signed integer to be unsigned.  Someone more familiar with the code should probably check to see if this results in the expected behaviour.
* Added .DS_Store to .gitignore so that it would ignore the files that OS X drops in every directory that I happen to open up in the Finder.

I have more information in the individual commit descriptions themselves.

Let me know if you want me to modify anything in these commits and I will resubmit with the requested modifications.

-Adam