Tweak git ignore and attributes (#618)

* Update .gitignore

Avoid unwanted system files.

* Create .gitattributes

Enforce preferred line ending style.

.gitattributes

@@ -0,0 +1,13 @@
+# Set the default behavior, in case people don't have core.autocrlf set.
+* text=auto
+
+# Files that will always have UNIX line-endings LF rather than Windows-style CRLF.
+*.css text eol=lf
+*.js text eol=lf
+*.md text eol=lf
+*.svg text eol=lf
+*.xml text eol=lf
+
+# Denote all files that are truly binary and should not be modified.
+# (binary is a macro for -text -diff)
+*.png binary