Add API reference documentation

This commit adds API reference documentation and updates README.md,
.gitignore and ubuntusetup.sh files accordingly.

This commit only includes the .rst source of the documentation. The
documentation can be built in a number of formats with Sphinx
using the Makefile provided.
Update version number to 0.2.1

This commits updates the version number to 0.2.1 in README.md and
icarus/__init__.py.