Fixes #26 - Implement static String routines to read data from hex

Four functions:
valueFromPrintableHex/valueFromPrintableHexW
bytesFromPrintableHex/bytesFromPrintableHexW

And a great deal of wrappers which make use of these in simpler
fashions, have each been implemented.

In addition, unit tests for each of these have been provided.

String code which touches wchar_t is now only compiled if
preprocessor token KTSDK_HAVE_WCHAR_T is not true. This is necessary
for systems that do not provide a wchar_t type, or if the user does
not wish to use it.
Merge pull request #26 from caitp/issue-26

[enhance] String routines to read uint32/uint16/bytes from printable hex