README: wrap at 79, improved formatting

Update the README doc to wrap at 79 characters where possible and
improve formatting to better match the way it will actually be rendered
with Markdown. For example:

One statement.
Another statement.

Is rendered with Markdown as:

One statement. Another statement.

Tested with grip markdown renderer.
REST template: 79 char wrap, formatting

Wrap the REST markdown template at 79 characters where possible. Remove
trailing whitespace.
Merge pull request #141 from mesosphere/doc-formatting-79char

README and REST documentation formatting improvement