Add explicit setup of expected namespaces

When connecting to an nrepl server, explicitly require clojure.pprint, and
use clojure.repl/doc.  When used with an nREPL server that didn't match
these assumptions, nrepl.el commands otherwise fail.
Add convenience bindings to clojure-mode-map for starting nrepl

Since people are generally in some Clojure buffer when starting
nrepl.el it's useful to have convenient keybindings for that in
`clojure-mode-map`. The mnemonic for the keybindings are `j` for jack-in
and `c` for connect. The keybindings are autoloaded and added to
`clojure-mode-map` after `clojure-mode` has been loaded.
Add multiple REPL connections

Allows REPLs for several different nREPL servers at once.

The default connection is used to dispatch nrepl commands in clojure buffers.
Use the nrepl-make-repl-connection-default command to make a specific repl
session the default.

Adds a nrepl-close command to close the current connection.

The current buffer is returned by the nrepl-current-connection-buffer function.
nrepl-connected-list maintains a list of connections.  When closing REPLS,
this is used to decide whether to disable nrepl interaction mode on clojure
buffers, and in order to set a new default connection if one is available.

Implements #75.
Add multi REPL readme section and changelog entry
Warn when requesting a repl matching an existing repl

Track the endpoint and project directory of connections, and warn if the user
tries to create a new repl that matches the endpoint or project-directory of an
existing repl.
New customization variable: nrepl-popup-stacktraces-in-repl

We use nrepl-popup-stacktraces only non-REPL buffers, and introduce a
specific flag for the latter.
Add travis checkdoc configuration

Note, the build currently fails.
Fix all checkdoc issues.
Merge pull request #262 from hugoduncan/feature/add-travis-checkdoc

Add travis checkdoc configuration
CHANGELOG and README entries for nrepl-popup-stacktraces-in-repl
Merge pull request #232 from bbatsov/clojure-mode-map

Add convenience bindings to clojure-mode-map for starting nrepl
Merge branch 'feature/add-aux-requires' of https://github.com/hugoduncan/nrepl.el into hugoduncan-feature/add-aux-requires
Checkdoc fixes.
Merge branch 'hugoduncan-feature/add-aux-requires'
Merge pull request #259 from jaor/local-popup

Allow local values for nrepl-popup-stacktraces
Fix remaining checkdoc errors.
Add TravisCI build status image to the README
Merge pull request #264 from bbatsov/add-travis-icon

Add TravisCI build status image to the README