New value & set-value! core functions

`value` & `set-value` are primarily used to get/set the values of form elements such as input, select and textarea; they act much the same way as other core functions: `text` / `set-text!` / `html` / `set-html!`

Also clarified readme docs on how to run tests.

All the test pass according to the generated html/js, but I see the following on the command line:

    $ lein cljsbuild test
    Compiling ClojureScript.
    Compiling "target/main.js" from ["src"]...
    Successfully compiled "target/main.js" in 13.820142752 seconds.
    Compiling "target/unit-test.js" from ["src" "test"]...
    Successfully compiled "target/unit-test.js" in 9.419794231 seconds.
    Running all ClojureScript tests.
    PElement: js/Window not defined by browser, skipping it... (running on phantomjs?)
    2 tests failed
    Exception in thread "main" java.lang.Exception: Test failed.
    	at cljsbuild.test$run_tests.invoke(test.clj:12)
    	at user$eval199.invoke(NO_SOURCE_FILE:1)
    	at clojure.lang.Compiler.eval(Compiler.java:6511)
    	at clojure.lang.Compiler.eval(Compiler.java:6501)
    	at clojure.lang.Compiler.eval(Compiler.java:6477)
    	at clojure.core$eval.invoke(core.clj:2797)
       	at clojure.main$eval_opt.invoke(main.clj:297)
    	at clojure.main$initialize.invoke(main.clj:316)
    	at clojure.main$null_opt.invoke(main.clj:349)
    	at clojure.main$main.doInvoke(main.clj:427)
    	at clojure.lang.RestFn.invoke(RestFn.java:421)
      	at clojure.lang.Var.invoke(Var.java:419)
    	at clojure.lang.AFn.applyToHelper(AFn.java:163)
    	at clojure.lang.Var.applyTo(Var.java:532)
    	at clojure.main.main(main.java:37)
    Subprocess failed

Ubuntu 13.04 64-bit, Lein 2.1.3, Jdk 1.7.0_21