Export Atom's version of React and Reactionary

Fixes #3101 and works around facebook/react#1939.
Show detailed React errors/warnings in devMode

Fixes #3099.

React development mode has a lot of useful warnings and exceptions. These don't
show up in production mode. This change puts React into dev mode when Atom is
run via `atom --dev`.
Upgrade to fs-plus 2.2.6
Upgrade to autocomplete@0.29
Upgrade to language-javascript@0.38
individual descriptions for instructions
Upgrade to language-javascript@0.39
Merge pull request #2680 from Bengt/patch-1

individual descriptions for instructions
:memo: Mark mdkeb step as optional
:memo: Remove step that is only is supported on Mac
:memo: Mention installed apm command
Base font styling on the computed style of the editor element

Previously, font styling was always explicitly assigned via the config.
This commit is the first step in basing the font styling of the editor
on the styles assigned via CSS. This will allow the editor's
font-family, font-size, and line-height to be assigned via CSS just like
they are for any other element, which will make it easier to style mini
editors.

We still need to switch the font settings to adjust a global stylesheet
rather than updating inline styles on each editor individually.
Only handle stylesheet changes after initial measurement
Handle editor font config options with a global stylesheet

Previously, each editor observed font-related config values on its own
and applied inline styles to honor them. This made it difficult to style
the editor like a normal element with CSS.

Moving this to a global stylesheet that targets editors via the .editor
selector means that the font size setting can be overridden in specific
contexts, such as when using mini editors.
Measure DOM in EditorComponent when a stylesheet is updated
Merge pull request #3104 from dmnd/react-dev-mode

Show detailed React errors/warnings in devMode
Merge pull request #3106 from atom/ns-react-css-font-styling

Apply React editor font styles via CSS instead of inline styles
Remove extra nextAnimationFrame call
Increase overdraw margin to avoid intermittent full screen repaints

On Chromium 35, the screen seems to be full-screen painting on a Cinema
Display every few frames, causing frame rates to drop from 60 to 30 when
autoscrolling with the cursor. Increasing the overdraw avoids this.
Upgrade to atom-shell@0.15.0