v2.26 merge

1. go errors implementation
2. changed IO functions to use []byte instead of string, get rid of bunch of unneeded copies
3. new logging - use go log.Logger and pass it to elliptics node
4. switched writer to use io.Reader interface
5. added preapre/write/commit functions and use them when io.Reader holds a very large object - it writes data using chunks. This methods will be changed a bit - it requires to cleanup returned json mess