Pragma once preprocessor directive support added.

Support for the non-standard but widely supported preprocessor directive
#pragma once. The script will remove all the pragmas from the included
content, except the root file.
Angled and quoted includes now differ.

File included with quotes is searched in the 'parent' file directory
first. This is complying to most compiler implementations.
Datetime placeholders in prologue file

Prologue file can now contain Python datetime strftime placeholders to
add amalgamation time to resulting file prologue.
Missing newlines at end of several test files
Fix some minor problems with pull request #1.

TranslationUnit._is_within() has been changed to an ordinary method,
it seemed to be slightly preferable to adding the @staticmethod decorator.

Replaced system.h with time.h in the test data, system.h is not a standard C
header and relying on its presence can therefore result in an unnecessary
test failure.
Merge branch 'alex-che-master'.