Remove duplicated using declarations
Add byte_buffer::operator=

in C++11, implicit definition of copy assignment operator
is deprecated if copy constructor and/or destructor is defined.
Add swap to byte_buffer
Remove byte_buffer's explicily defined special member functions

They are trivial, so using implicitly defined one is better.
Comment for byte_buffer
Fix implementation of byte_buffer::assign
Add test for byte_buffer
&(*buf_)[0] is undefined if buf_->size() == 0
Merge pull request #366 from gintenlabo/remove-duplicated-usings

Remove duplicated using declarations
Merge pull request #367 from gintenlabo/fix-byte_bufffer

Fix core::common::byte_buffer
Approve cpplint

except #include <algorithm> (which is not necessary)
Fix typo

this commit is unconcerned from current branch,
but it's too trivial to make a topic branch.
Add `// NOLINT'
Refactor sort_and_merge
Fix sort_and_merge

Assignment is better than swap in this case
Optimize sort_and_merge futher

If vector's size is 1, no process is required.
Merge pull request #371 from gintenlabo/refactor-sort_and_merge

Refactor sort_and_merge
Merge pull request #368 from gintenlabo/fix-cpplint-violations

Fix cpplint violations
Remove key_manager::vacant_
Make key_manager's operator= implicitly-defined