Removed lens memoization
Removed multiple arguments support from .attr() and .partialAttr()
Added resolvers

Resolver is a thing defining lens error handling.
This is done by inversion of control:
- lens semantics are provided by check, get and set functions
- resolver combines them and controls how check influences get and set
behaviour
Updated docs

- no multiargument .attr()
- .traverse() -> traversals()
Merge pull request #10 from Suor/resolvers

Resolvers