Adds support for PostgreSQL which resolves #87 and is mentioned in #246.

This includes migrations as well as Dockerfile's for amd64.

The biggest change is that replace_into isn't supported by Diesel for the
PostgreSQL backend, instead requiring the use of on_conflict. This
unfortunately requires a branch for save() on all of the models currently
using replace_into.

Cargo.toml

@@ -14,6 +14,7 @@ build = "build.rs"
 # Empty to keep compatibility, prefer to set USE_SYSLOG=true
 enable_syslog = []
 mysql = ["diesel/mysql", "diesel_migrations/mysql"]
+postgresql = ["diesel/postgres", "diesel_migrations/postgres", "openssl"]
 sqlite = ["diesel/sqlite", "diesel_migrations/sqlite", "libsqlite3-sys"]
 
 [target."cfg(not(windows))".dependencies]
@@ -105,6 +106,9 @@ handlebars = "2.0.2"
 soup = "0.4.1"
 regex = "1.3.1"
 
+# Required for SSL support for PostgreSQL
+openssl = { version = "0.10.24", optional = true }
+
 # URL encoding library
 percent-encoding = "2.1.0"