Reference exception in rack.errors
Add test for exception with custom #to_s method
Add custom error message and exception backtrace to `rack.errors`
Simplify building exception message

Also fixes problem with exception without message, in such case class
and message are equal
Pretty print exceptions in rack.errors
Merge branch 'zlw-rack_errors'
Removed Lotus::Action::Throwable#throw in favor of #halt
[ci skip] Changelog
[ci skip] LICENSE.txt -> LICENSE.md
Gem: build only with lib/ and essential files
[ci skip] README
Updated README documentation
Lotus::Controller: .duplicate => .dup, .generate => .duplicate
[ci skip] Documentation for Lotus::Controller and Lotus::Action
Controller.duplicate can accept nil controllers namespace
[ci skip] Documented all the Lotus::Action modules
[ci skip] Documentation for Lotus::Controller modules
[ci skip] Yardoc adjustments
Bump lotus-utils dependency to 0.2.0
Bump version to v0.2.0