gitignore './build' and egg-stuff

So that building the package does not make the repo '-dirty'
when used as a submodule.

Copied from the nifty .gitignore.

.gitignore

+# from https://github.com/github/gitignore/blob/master/Python.gitignore
+
+# Byte-compiled / optimized / DLL files
+__pycache__/
+*.py[cod]
+*$py.class
+
+# C extensions
+*.o
+*.so
+
+# Distribution / packaging
+.Python
+env/
+build/
+develop-eggs/
+dist/
+downloads/
+eggs/
+.eggs/
+lib/
+lib64/
+parts/
+sdist/
+var/
+*.egg-info/
+.installed.cfg
+*.egg