Add support for GnuPG2/GPG2.exe

[blarb]

I think you're using the starksoft GPG-wrapper as it was used in the original OutlookGnuPG. After a correspondence some weeks ago the wrapper (http://www.starksoft.com/prod_openpgp.html) has been updated to support gpg2.exe by its developer. Perhaps you could include the updated version in the project as there are some compilations like gpg4win (http://www.gpg4win.de) which are using GPG2 in the recent versions. I didn't look at the source of the new wrapper but i think it should be easy to update to the current version.

[doegox]

From what I could see, all gpg options currently used are also defined into gpg2 and the output looks also the same. This seems confirmed also by the gpg4win patch for winpt. So a priori it would be enough when looking for gpg.exe to first look for gpg2.exe and if not found in the provided directory, look for gpg.exe.

Note that it seems there is some confusion in the current code on gpg paths, see #22 which should be fixed first.

Some more insights after a first set of tests:

• GPG2 is obeying regional linguistic settings. To force English in a first step, one had to export LANG=C in the environment of the cmd.exe running gpg2.exe. For a better support see issue #36
• GPG2 can rely on its gpg-agent rather than giving the passphrase through OutlookGnuPG, see issue #37
• Even when the plugin provides the passphrase, GPG2 still tries to call its gpg-agent which therefore must be accessible from the PATH. One may probably complete the PATH in the environment of the cmd.exe running gpg2.exe (to be checked)

[twalrant]

Partially supported, plugin attempt to use gpg2.exe first than gpg.exe and LANG=C is set whenever gpg2.exe is used.