Add optional author line in header

The goal is to trigger the display of the title, currently missing
from github preview.
Add extra characters to underline to get title displayed in github preview
Convert Markdown link to asciidoc

Reference:
http://www.methods.co.nz/asciidoc/userguide.html#_urls
Convert ruler (hr) line from markdown to asciidoc

"A line of three or more apostrophe characters will generate a ruler line."

Reference:
http://asciidoc.org/userguide.html#_rulers
Decrement level of all titles, except main title

The rationale is to keep the level 0 for the title of the document,
and start other titles at level 1.

The example in section 11.2 One line titles shows that level 0
is the document title, while levels 1 to 4 correspond to sections
of the document.

Reference:
http://asciidoc.org/userguide.html#X17
Use lines of 80 characters as delimiters for code blocks

It appears that there is a conflict between dashes used as level 1 title
and dashes used as delimiter for blocks, at least in the preview on github.

Using longer lines as delimiter for code blocks makes it unlikely
that the underline of a title may have the same length.
Merge branch 'i18n' into asciidoc

Conflicts:
	dataflow.asc
Replace two-line titles with one-line titles

This is an attempt to get titles rendered as titles in github preview,
rather than the start of a code block...
Remove ruler delimiters around introduction

Using one-line titles lead to a correct rendering of titles in github preview,
except for the top-level title. This is an attempt to fix this issue.
Revert "Remove ruler delimiters around introduction"

This reverts commit c26075fea7479a261cab3f3e921d3ecef8b64376.

This had no impact.
Replace top-level title with level 1 title, as a turn-around
Translate two first subsections of Needs (Besoins)
Change "Needs" into "Requirements", that fit better with the section purpose
Remove ruler lines around introduction (superfluous)
Replace 'assigned' with 'updated with' (closes issue #6)

Summary of the discussion: while a relationship without transformation
results in an assignment, 'update' covers both cases (simple assignment
or transformation using the new value as input), which is more consistent.
Translate section 'Connexion + fonction de transformation'
Translate section 'Connexion n - m (demux / mux)

In the course of this translation, I noticed two issues:

* the role of components A and B is ambiguous:
  - which one is the input?
  - which one is the output?

  The formulae are unambiguous, but the text should be too. As a sign of this
  confusion, the index 'n' used for component A in the text is used for the
  properties of component B in the first formula.

* the formulae for the multiplexer and the demultiplexer were reversed:
  - mux (not demux) is one resulting from f ( several )
  - demux (not mux) is several resulting from f ( one )

Reference:
http://en.wikipedia.org/wiki/Multiplexer
Rename variables used in formulae to clarify inputs and outputs

The components are now c1 and c2 instead of A and B,
in the order of the sequence c1 -> c2 in the data flow.

The output properties (of component c1) are named 'out' followed with the
index, from 1 to N.

The input properties (of component c2) are named 'in' followed with the index,
from 1 to N (when identical) or 1 to M.
Quote all variables when used in the text for consistency.

Text in single quotes is displayed in italics, which makes it stand out.
It was strange to have the same variable 'c1' or 'out1' sometimes in italics
and sometimes in roman.
Translate stub sections 'Déconnexion'