Add default lenses
Always set in fallback resolver

Since place (attr, index or whatever) has default
then it's ok if it's assigned something.
Change .attr() and .at() interface to include defaultValue

- .attr() and .at() now behave as default lenses if provided second
argument
- use new low-level .makeAttrLens(name, resolver) and
.makeAtLens(i, resolver) to make a lens with custom resolver
Merge pull request #11 from Suor/master

Default lenses