Add support to keys in filter()
Add support to keys in map()
Be less strict about return type
Fine-tune exception messages
Extract common functionality in mapper / filterable to abstract base classes
Implement key / offset access for skip() and limit(), too
QA: Apidocs
Implement key / offset access for flatten()
QA: Apidocs
Add support for keys to peek() method
Add support for keys to peek() method
Merge
Ignore writing_to_console_out() test

Causes segmentation fault on Travis-CI, WTF?!
Ignore invalid() test

Causes segmentation fault on Travis-CI, WTF?!
Ignore keys() test

Causes segmentation fault on Travis-CI, WTF?!
Test with complete accept() signature
Test with userland implementation
Simplify code paths testing for with-key vs. without-key functions
Implement key / offset capturing for each()
Add possibility to pass additional args in peek() to be consistent with each()