Only widen the gutter when showGutter is enabled

After installing linter the gutter becomes wider. Even when the config option
showGutter is disabled the gutter is still wider than it should be. This change
only adds gutter CSS when showGutter is enabled.

Test Plan:
Toggled showGutter on and off and saw gutter change width
oops, forgot to fix lint errors after recording gif :)
Fix for multiple linters on the same file (#139)
Fix typo: hightlight -> highlight

Test Plan:
Crossed fingers
better way to assemble path
Update README.md
Use decorations API to display gutter markers

This new API is designed exactly for our use case. One benefit is that
`GutterView.render` is now no longer called hundreds of times a second (!)

I copied most of the implementation from the git-diff package.

Test plan:

Opened a new editor and verified that gutter markers appear and disappear when
lint errors are caused and resolved. Also added a logging statement to
`GutterView.render` to verify that it's now only called when the file is
linted.
Use decorations API to display highlight markers

Again the implementation is inspired by the git-diff package.

Test Plan:
Opened a new editor and verified that highlights show up. I had to tweak the
indexing for columns after noticing that errors on the final line didn't show
up after this change.
Consolidate markers into `LinterView`

Since `GutterView` and `HighlightsView` were basically the same thing, I
extracted out the shared code to `LinterView` and deleted them.

Test Plan:
Opened a file with lint errors. In another Atom window, went to settings and
turned gutter and highlights on and off and made sure that these settings still
updated in the other window.
Revert to getNodeExecutablePath

Due to an issue in Atom, BufferedNodeProcess does not produce any output, resulting in no linting occurring. For now, revert back to BufferedProcess and getNodeExecutablePath
Fix existing tests and add a new one
Add keybinding to trigger linting manually

Default keybinding copied from the Sublime Text linter.

Fixes #146.

Test Plan:

  * Opened Atom, turned off lint on focus/modify/save
  * Reloaded the window and edited code so that there was a lint error
  * Pressed `cmd-ctrl-l` and saw a lint error appear
  * Fixed the lint error and pressed the bind again
  * Lint error now gone
Merge pull request #141 from dmnd/fix-typo

Fix typo: hightlight -> highlight
Merge branch 'master' into decorations-api

Conflicts:
	lib/linter-view.coffee
Merge pull request #147 from dmnd/decorations-api

Use decorations API
Merge pull request #142 from stormslowly/master

better way to assemble path
fix typo (showHightlighting -> showHighlighting)
update CHANGELOG
Prepare 0.5.0 release
Merge pull request #149 from dmnd/add-key-binding

Add keybinding to trigger linting manually