extract_files(): security fix.

The names of the extracted files where not checked for the special
directory "..". This would allow a malicious user to successfully
perform a path traversal attack.
extract_files(): add support for directories.

The filenames in a request are now treated as paths. Thus, directories
may be created and the files may be placed inside them accordingly.
Directories in libraries are not yet supported, but this is the first
step.

Also fixed a small grammatical error in the documentation.
Mimic Arduino IDE on handling include directives.

The Arduino IDE does not differentiate between the <> and "" characters
in include preprocessor directives. This is done to facilitate library
usage by novice programmers.

Spoiler alert: the utility directory present in some libraries will be
fully supported in the future.
Add support for utility directory.

Some libraries contain a special 'utility' directory. Up to this point,
files had to be moved to the parent directory and include directives had
to be changed. This is no longer needed.
Merge branch 'untainted'

Conflicts:
	compiler.php