README.md: update links

* Gittip was renamed to Gratipay.
* Against all odds we made a 1.2.1 maintenance release.
* To help hyperlink-challenged users the Known Issues
  have been merged into the FAQ.
README.md: wording changes
Merge branch 'master' into next
README.md: update dependency information
README.md: remove non-Git installation instructions

The only way to install from the `next' branch is to build it from
the Git repository.  Remove all other installation instructions, they
do not apply to the development branch.  Before that is being merged
into `master', new instructions will be written.