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.

key.hpp

@@ -35,6 +35,7 @@
 #include <functional>
 #include <stdint.h>
 #include <iosfwd>
+#include <string>
 
 enum {
 	HMAC_KEY_LEN = 64,
@@ -67,6 +68,8 @@ public:
 	bool				load_from_file (const char* filename);
 	bool				store_to_file (const char* filename) const;
 
+	std::string			store_to_string () const;
+
 	void				generate ();
 
 	bool				is_empty () const { return entries.empty(); }