Move back to optional dependency for the dagger-compiler.

This reverts commits 2014bce8e79f7bdba4fcaa4f581e6189543ded00
and f784b96520d0a408ac76b5b187e7bb9dc5c32ff1, with a few other
changes in the documentation and IT tests added afterwards.

Closes #194
Merge pull request #198 from tbroyer/dagger-compiler-optional

Move back to optional dependency for the dagger-compiler.