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.cpp

@@ -36,6 +36,7 @@
 #include <fstream>
 #include <istream>
 #include <ostream>
+#include <sstream>
 #include <cstring>
 #include <stdexcept>
 
@@ -146,6 +147,13 @@ bool		Key_file::store_to_file (const char* key_file_name) const
 	return true;
 }
 
+std::string	Key_file::store_to_string () const
+{
+	std::ostringstream	ss;
+	store(ss);
+	return ss.str();
+}
+
 void		Key_file::generate ()
 {
 	entries[is_empty() ? 0 : latest() + 1].generate();