Publish userguide to http://docs.bitsquare.io

Documentation generated from AsciiDoc sources in the `docs` directory
may now be deployed to docs.bitsquare.io via SSH using

    $ ./gradlew publish

Doing so requires supplying the `bitsquareDocsPassword` property to the
Gradle build. This can be done in one of three ways:

 1. ./gradlew publish -PbitsquareDocsPassword=...

 2. Add a `bitsquareDocsPassword=...` entry in ~/.gradle/gradle.settings
    This is the recommended approach when publishing locally.

 3. Provide an environment variable in the form
    `ORG_GRADLE_PROJECT_bitsquareDocsPassword=...`. This is the approach
    used in the Travis CI build (see Notes below).

The Travis CI build now calls `gradle publish` following each successful
build, meaning that documentation is now continuously published and made
available at http://docs.bitsquare.io.

Notes:

 - Regarding the Gradle SSH plugin, see:
   https://gradle-ssh-plugin.github.io/ and
   https://plugins.gradle.org/plugin/org.hidetake.ssh

 - Regarding the use of the `plugins { }` closure in build.gradle (a new
   feature in Gradle 2.1), see:
   http://www.gradle.org/docs/current/release-notes#easier-use-of-community-plugins

 - Regarding the use of the `after_success` hook in .travis.yml, see:
   http://docs.travis-ci.com/user/deployment/custom/

 - Regarding the use of encrypted environment variables in Travis CI, see:
   http://docs.travis-ci.com/user/environment-variables/#Secure-Variables.
   In particular, see the use of the `travis` gem and `travis encrypt`.

 - Regarding setting Gradle project properties via ORG_GRADLE_PROJECT_*
   environment variables, see:
   http://www.gradle.org/docs/current/userguide/tutorial_this_and_that.html#sec:gradle_properties_and_system_properties

Resolves #169
WIP: userguide
WIP: userguide (test timestamp)