Provide downloadable binaries

[lukecyca]

To make it easier for non-developers to use this tool, we should provide instructions to download a pre-compiled binary in the README.

We could use http://gobuild.io, or perhaps http://drone.io.

[donovanhide]

ls -alrth bin/tx
-rwxr-xr-x 1 donovanhide staff 8.8M Jul 28 21:13 bin/tx

Looks like the binaries for OSX, Windows and Linux would exceed the 10MB drone.io artefact limit, which is a shame. Wish Go binaries were smaller!

[donovanhide]

Obviously we could just zip them up :-)

ls -alrth bin/tx*
-rwxr-xr-x 1 donovanhide staff 8.8M Jul 28 21:13 bin/tx
-rw-r--r-- 1 donovanhide staff 2.4M Jul 30 16:23 bin/tx.gz