Add postgresql support in docker deployed.

swap \n for \000, update len check, when reading receive-pack input

Add the instruction of installing gogs with PostgreSQL

Add the support of postgres as the database.
(Config the db in the build.sh file, make the DB_TYPE to 'postgres').

Replace the host with $DB_PORT, which is dependent to the database imagae's exposed port.

Replace the 'mysql' options with 'db' options, makeing the config according to the database type.

change gravatar image to match URL scheme

update SetEngine for postgresql #76