Cpp: fixed write_data by chunks.
Fixed prepare size - for write with offset it should includes offset and file size
Cpp: Updated comments.
Cpp: Extended comments and notes about write_prepare, write_plain and write_commit.
Merge pull request #246 from shaitan/clean

Cpp: fixed write_data by chunks.
Merge branch 'master' of github.com:reverbrain/elliptics into python