Support for extracting certificates from digitally signed binaries

A compact overview of the most important commits on this branch, in reverse order:

8a2a85fbf1  pesec: Show all signers and whether certificate signature is valid.
600505018f  pesec: Support new CLI parameters --certoutform and --certout.
1d007537fe  Fix sign conversion warning.
8fa06d374a  Parse certificates from digitally signed PE's.
e9accc498d  README now contains instructions on how to build in Mac OS X.
8f185fe43e5  libpe: Fix missing cert types.
                         pesec: Handle unsupported/unknown values, and adjust printed data.
f8ace9ef656  libpe: Fix indentation and line-breaks.
fcfe9461984  libpe: Add very basic support for the security directory.
bbe12bd663  libpe: Introduce new function pe_get_data_directory().
46a81534b8  Fix possible memory leaks in pe_get_directories().

Please, review those commits prefixed with "libpe" as they replace some #defines with enums, introduce new APIs, etc.
I felt it would be good to split the headers in order to avoid code duplication (mainly lib/libpe/types.h). Also though it would be positive to have separate headers for specific directory types (example, lib/libpe/dir_entry_security.h), so we don't clutter libpe.h with specifics.

I've tested only on Mac OS X, so I'd love some feedback from various users and systems.

Hope everyone enjoys it.