Fixes #25 - Add static string routines to output printable hex

New String static methods:

toPrintableHex32
toPrintableHex16
toPrintableHex8
toPrintableHexW32
toPrintableHexW16
toPrintableHexW8

May implement a 64 bit version in the future.
Merge pull request #25 from caitp/issue-25

[enhance] String routines to output printable hex