Integrate User Guide as in-app help system

[cbeams]

As mentioned at #63 (comment), the plan is to rework the Bitsquare UI to link to How-To sections specific to the screen or activity that the user needs help with.

The docs shall published on a version-by-version basis, so regardless the version of Bitsquare the user is using, they'll always be referred to the correct documentation. See #169 for details (which this issue depends on in order for work to begin).

[ManfredKarrer]

To use a simple web link and let it open in the browser seems the better solution instead of a an in-app help. We wull use http://docs.bitsquare.io/ and need to customize the anchors to the sub sections.