Document histograms in the exposition format and new storage flags.

Adjust rule checker docs to changed behavior

Link to "Client Library Guidelines" doc.

Link to new exporters and client libraries.

Document exposition format directly on the website.

This only documents the latest 0.0.4 exposition format version, with a
link to the legacy doc at the bottom. I also took the liberty to
fix up/adapt some of the content. Tables from the original doc are
transliterated as nested lists instead, because Markdown doesn't support
lists in tables (and the old doc made heavy use of them). A future
option could be to go for completely manual HTML tables with HTML lists
in them.

This fixes https://github.com/prometheus/docs/issues/28

Add documentation for exp, ln, log2, and log10 functions.

Update benchmarks.

Remove duplicate sentence

Once my PRs are in and 0.13.0 is released, we can merge this to
reflect the better performance.

Fix usage of include labels in group modifiers.

Update roadmap items

The final change to the group modifiers requires included labels
to be on the "many"-side of the operation. This adjusts the example
and description to match this behavior.

Document histograms and histogram_quantile.

Most importantly, created a completely new section for histograms and
summaries and updated all the references.

Also add other minor improvements.

Documentation for vector matching added.

Adjust docs to existing standards.

- "Prometheus'" to "Prometheus's"
- contractions removed
- no title case in headings

Random docs improvements

GH wiki is not available anymore

add mesos_exporter to list of exporters

CONTRIBUTORS.md is gone, contributors are being listed in AUTHORS.md.