Better error messages for the namespace argument

Work around when the first arg is -1

When seemingly valid arguments are given for non existent classes,
return a meaningful error message.
Described *1-*3,*e in repl-welcome

I have learned only by chance about the fact that REPL stores evaluation results in *1 - *3 and the last exception in *e. I am certainly not the only one unaware of this so it would be good to include it in the welcome message.

(For random visitors: they are defined in clojure's [`./src/clj/clojure/main.clj`](https://github.com/clojure/clojure/blob/master/src/clj/clojure/main.clj).)
Add alias explanations for `lein help task`.

Closes #1316.
Don't add alias-help as public API in lein.help.

Also add in a couple of tests to ensure that aliases print docstrings.
Added support of maven plugins in project.clj.

It's possible to add definition :plugins in project definition.
Example: 
 :plugins [[com.theoryinpractise/clojure-maven-plugin "1.3.13"]
           [org.apache.tomcat.maven/tomcat7-maven-plugin "2.1"]]
Maven plugins are now supported by :pom-plugins to avoid name conflict
with Leiningen :plugins vector. 

Added maven <configuration/> tag inside of plugin definition.

Added usecase of :pom-plugins in sample.project.clj
Fix tutorial repl message. Addresses #1317.
Use more descriptive local name.
Use profile-scoped targets in app template.
Don't warn about user profiles when running outside a project.
Warn when nrepl dependency isn't found. Fixes #1330.
Fix typo in lein template invocation command.
Merge pull request #1332 from jconti/master

Fix typo in lein template invocation command.
Better error message for bad -m args.
More descriptive error message for nonexistant ns.

Closes #1257.
Merge in better error messages for -m in lein run.
Fallback to .nrepl-port for 'repl :connect' subtask
Merge pull request #1336 from jcrossley3/connect-via-nrepl-port-file

Fallback to .nrepl-port for 'repl :connect' subtask
Abort with a nicer message when neither port nor .nrepl-port is provided
Merge pull request #1338 from jcrossley3/port-required

Abort with a nicer message when neither port nor .nrepl-port is provided