add ability to specify custom exception handlers

change approach in exception handlers

- Allow only method name or status code as handler
- Don't require to return array with code and body

use handler as void function

Another change in exception handling apporach

- Remove `exception_code` in favour of `exception_handler` in Configuration
- Add Thorwable::Handlers, which defines handlers for all status codes
- Remove ability to specify handlers by status code in favour of explicit handlers

Rename class name to reflect recent changes

ruby 2.0.0 compatibility changes

Revert "Another change in exception handling apporach"

This reverts commit 3ee362073fdc3a080a7eee41a300e351f234db08.

Revert "Rename class name to reflect recent changes"

This reverts commit e3c9d82fa3a287f2d9e752088e79261314abb942.

Revert "ruby 2.0.0 compatibility changes"

This reverts commit 77294157e99baab5816f3e36f2341a04ec6ed356.

small code cleanup

Don’t use protected handled_exceptions method

Rename `exception_code` to `exception_code` which returns either exception code, or handler name
Merge branch 'krzyzak-exception-handling'