Update roadmap items

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.

Remove extra double-quotes

Update docs with regard to instance label change.

Make the benchmark results more precise.

I have run a number of benchmarks by now. Results depend on a lot of
boundary conditions, sometimes in surprising ways. But I guess that's
true for any benchmark... Anyway, the text now gives at least one data
point to get an impression of what is doable.

Value is in the sample, not the labels.

Add note about memory usage during build.

Add Python client

add annotations documentation.

Fix regex, mention reserved labels.

Point out README.md build section from tutorial.