build.xml: Add csg shortcut for compile-source-gen

‘ant clean’ (occasionally necessary when the IDE interferes with the ant
build) regularly wipes out source-gen, and it’s annoying to write
“compile-source-gen” every time, even with tab completion.
Atom: Update documentation link

The type ended up being called QualifiedExpression rather than
MemberExpression (I’m pretty sure it didn’t exist yet when I wrote that
documentation).
valueExpressionToCeylon: Support for Operations

I have no idea why none of the tests caught that… at least the
stringTemplate test (later commit) did.
Add StringTemplate + stringTemplate utility function

Closes #41.
Add tests for StringTemplate backend conversion

This exposed the bug fixed in cb44440.

CC #41.