Use panic-halt crate, panic-abort requires nightly toolchain features

When trying to follow the Getting Started->Building instructions, the
`panic-abort` crate throws the following build error on rust stable
version 1.33:

```
   Compiling panic-abort v0.3.1
error[E0554]: #![feature] may not be used on the stable release channel
  --> /home/mike/.cargo/registry/src/github.com-1ecc6299db9ec823/panic-abort-0.3.1/src/lib.rs:22:1
   |
22 | #![feature(core_intrinsics)]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0554]: #![feature] may not be used on the stable release channel
  --> /home/mike/.cargo/registry/src/github.com-1ecc6299db9ec823/panic-abort-0.3.1/src/lib.rs:23:1
   |
23 | #![feature(panic_handler)]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0554`.
error: Could not compile `panic-abort`.

To learn more, run the command again with --verbose.
```

Switch to the `panic-halt` crate that builds on stable and provides the
`#[panic_handler]` function necesary to get a minimal successful build.

src/getting-started/01.00.BUILD.md

@@ -116,7 +116,7 @@ If you have forgotten how to do this, try looking at [the cargo book][cargo].
 ``` rust
 #![no_std]
 
-extern crate panic_abort;
+extern crate panic_halt;
 
 fn main() {
 }
@@ -141,8 +141,8 @@ neither the crt0 nor the rust runtime are available,
 so even implementing `start` would not help us.
 We need to replace the operating system entry point.
 
-You could for example name a function after the default entry point, 
-which for linux is `_start`, and start that way. 
+You could for example name a function after the default entry point,
+which for linux is `_start`, and start that way.
 Note, you would also need to disable [name mangling][nm]:
 
 ``` rust
@@ -232,11 +232,11 @@ and cargo will automatically add `--target thumbv6m-none-eabi`.
 
 ### `src/main.rs`
 
-``` rust 
+``` rust
 #![no_std]
 #![no_main]
 
-extern crate panic_abort;
+extern crate panic_halt;
 
 use cortex_m_rt::entry;
 
@@ -271,7 +271,7 @@ An easy way to implement this is to use an infinite loop.
 #![no_std]
 #![no_main]
 
-extern crate panic_abort;
+extern crate panic_halt;
 
 use cortex_m_rt::entry;
 

src/getting-started/Cargo.toml

@@ -3,6 +3,6 @@ name = "start"
 version = "0.2.0"
 
 [dependencies]
-panic-abort = "~0.3"
+panic-halt = "~0.2"
 microbit="~0.6"
 cortex-m-rt="~0.6"

src/getting-started/src/main.rs

@@ -3,7 +3,7 @@
 
 extern crate cortex_m_rt;
 extern crate microbit;
-extern crate panic_abort;
+extern crate panic_halt;
 
 use cortex_m_rt::entry;