Add changelog and metadata for 0.7.3 (#167)

Cargo.toml

@@ -1,16 +1,7 @@
 [package]
 name = "tiff"
-version = "0.7.2"
-authors = [
-    "ccgn",
-    "bvssvni <bvssvni@gmail.com>",
-    "nwin",
-    "TyOverby <ty@pre-alpha.com>",
-    "HeroicKatora",
-    "Calum",
-    "CensoredUsername <cens.username@gmail.com>",
-    "Robzz",
-    "birktj"]
+version = "0.7.3"
+authors = ["The image-rs Developers"]
 license = "MIT"
 description = "TIFF decoding and encoding library in pure Rust"
 repository = "https://github.com/image-rs/image-tiff"