Support downloading from other Maven repositories

Developers may now request buck to use a local mirror by setting
the URL into local.properties, a file already ignored under the
current buck build process, e.g.:

  download.GERRIT = http://nexus.my-company.com/
  download.MAVEN_CENTRAL = http://nexus.my-company.com/

To support this usage buck now passes to download_jar.py only the name
of the repository in the URL.  download_jar parses local.properties
(if present) and replaces the "MAVEN_CENTRAL:" prefix with the URL
supplied by the user.

Because the URL can vary cache entries in buck-cache are now using the
SHA-1 we expect/verify against, instead of the SHA-1 of the URL.  This
makes it easier to find a specific JAR in the cache.  The artifact,
version and SHA-1 as named in the maven_jar() rule are the strings
used in the file name in buck-cache.

If no SHA-1 verification happens (e.g.  source attachment JARs that
are not included into the build) then sha1(canonical_url) is used.
Here the canonical URL uses the repository id prefix string, e.g.:

  MAVEN_CENTRAL:org/apache/mina/mina-core/2.0.5/mina-core-2.0.5-sources.jar

Change-Id: I5a469ab15fd36b81bf76f6b51e1110dfdcf04c86
diff --git a/Documentation/dev-buck.txt b/Documentation/dev-buck.txt
index f97fe6e..dfa19dc 100644
--- a/Documentation/dev-buck.txt
+++ b/Documentation/dev-buck.txt
@@ -136,11 +136,6 @@
   buck build download
 ----
 
-Currently there is no way to download dependencies other than over an HTTP
-connection from the source sites (Maven Central and the Gerrit storage
-bucket).  It is not yet possible to use, for example, a corporate Nexus
-proxy.
-
 When downloading from behind a proxy (which is common in some corporate
 environments), it might be necessary to explicitly specify the proxy that
 is then used by `curl`:
@@ -149,6 +144,15 @@
   export http_proxy=http://<proxy_user_id>:<proxy_password>@<proxy_server>:<proxy_port>
 ----
 
+Redirection to local mirrors of Maven Central and the Gerrit storage
+bucket is supported by defining specific properties in
+`local.properties`, a file that is not tracked by Git:
+
+----
+  echo download.GERRIT = http://nexus.my-company.com/ >>local.properties
+  echo download.MAVEN_CENTRAL = http://nexus.my-company.com/ >>local.properties
+----
+
 
 Build Process Switch Exit Criteria
 ----------------------------------