Add include guards to header file

Including this library in another library can cause double declaration compile errors, due to the whacky Arduino build process.

This change adds include guards to the header file. This prevents the header from being included more than once.

More about include guards: http://stackoverflow.com/questions/1653958/why-are-ifndef-and-define-used-in-c-header-files
Merge pull request #11 from cloudwalking/master

Add include guards to header file