Added a FsStats.total section that sums up all stats for the different path.dirs. Added `disk_io_size` and `disk_io_op` to the toXContent output, summing up `disk_read_size+disk_write_size` and `disk_writes+disk_reads` respectively.

Closes #4344 , Closes #4343
remove help command, list all cat commands in /_cat?h endpoint
Cat: Add nodes column descriptions.
Cat: Add descs for rest of actions.
Fix _all boosting.

_all boosting used to rely on the fact that the TokenStream doesn't eagerly
consume the input java.io.Reader. This fixes the issue by using binary search
in order to find the right boost given a token's start offset.

Close #4315
Introduce CHANGES.txt

In order to easier create release notes, we should switch to a CHANGES.txt file.

The problem with commits is, that they are written, when the developer is
kneedeep in code, which results in more likely technology focused commits
instead of feature/functionality focused commit messages. This file should
be used to explain, what really happened and what has been fixed from a
user point of view.