Writer class is added
Auxiliary methods were added into writer
Writer::write_impl changes state
Deferred_function class is added

Deferred_function class allows to create function that will execute:
1) once,
2) After it is called N times (N is a parameter of deferred_function).

That class represents a mechanism of joining asynchronous operations.
File is removed after any write failure
File is removed from all groups
File will be removed during simple upload in case of error is ocurred
Any writer_t error is writer_error
Namespace elliptics::writer is moved into elliptics
Buffered_writer class is added
Files will be removed during multipart upload in case of error is ocurred
Descriptions for each writer error code were added
Descriptions for each buffered_writer error code were added
Error handling explanation was added into upload_simple
Necessary comments were added into upload_multipart
Useless commented code was removed
Unused code was removed
Merge branch 'feature-mds-430-v2' into development
Merge branch 'development' into release-0.80
Version up to 0.80.3