Lighthouse v0.2.0 (Medalla) (#1452)

## Issue Addressed

NA

## Proposed Changes

- Moves the git-based versioning we were doing into the `lighthouse_version` crate in `common`.
- Removes the `beacon_node/version` crate, replacing it with `lighthouse_version`.
- Bumps the version to `v0.2.0`.

## Additional Info

There are now two types of version string:

1. `const VERSION: &str = Lighthouse/v0.2.0-1419501f2+`
1. `version_with_platform() = Lighthouse/v0.2.0-1419501f2+/x86_64-linux`

(1) is handy cause it's a `const` and shorter. (2) has platform info so it's more useful. Note that the plus-sign (`+`) indicates the the git commit is dirty (it used to be `(modified)` but I had to shorten it to fit into graffiti).

These version strings are now included on:

- `lighthouse --version`
- `lcli --version`
- `curl localhost:5052/node/version`
- p2p messages when we communicate our version

You can update the version by changing this constant (version is not related to a `Cargo.toml`):

b9ad7102d5/common/lighthouse_version/src/lib.rs (L4-L15)

beacon_node/src/cli.rs

@@ -1,11 +1,5 @@
 use clap::{App, Arg};
 
-// Default text included in blocks.
-// Must be 32-bytes or will not build.
-//
-//                              |-------must be this long------|
-const DEFAULT_GRAFFITI: &str = "sigp/lighthouse-0.1.2-prerelease";
-
 pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
     App::new("beacon_node")
         .visible_aliases(&["b", "bn", "beacon"])
@@ -248,9 +242,11 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
         .arg(
             Arg::with_name("graffiti")
                 .long("graffiti")
-                .help("Specify your custom graffiti to be included in blocks.")
+                .help(
+                    "Specify your custom graffiti to be included in blocks. \
+                    Defaults to the current version and commit, truncated to fit in 32 bytes. "
+                )
                 .value_name("GRAFFITI")
-                .default_value(DEFAULT_GRAFFITI)
                 .takes_value(true)
         )
 }