Initial GPG support

Run 'git-crypt add-collab KEYID' to authorize the holder of the given
GPG secret key to access the encrypted files.  The secret git-crypt key
will be encrypted with the corresponding GPG public key and stored in the
root of the Git repository under .git-crypt/keys.

After cloning a repo with encrypted files, run 'git-crypt unlock'
(with no arguments) to use a secret key in your GPG keyring to unlock
the repository.

Multiple collaborators are supported, however commands to list the
collaborators ('git-crypt ls-collabs') and to remove a collaborator
('git-crypt rm-collab') are not yet supported.

Makefile

@@ -3,7 +3,7 @@ CXXFLAGS := -Wall -pedantic -ansi -Wno-long-long -O2
 LDFLAGS := -lcrypto
 PREFIX := /usr/local
 
-OBJFILES = git-crypt.o commands.o crypto.o key.o util.o
+OBJFILES = git-crypt.o commands.o crypto.o gpg.o key.o util.o
 
 all: git-crypt