Unify a list of available backends
Adjust phrasing in 'switch' sub-command

'switch' jumps between existing builds (some backends of certain
versions), or things previously built via 'build' command.
Fix warns and suggest options when 'build' run w/o arguments

It used to shit warnings:

    $ rakudobrew build
    Use of uninitialized value $impl in string eq at /home/mvuets/.rakudobrew/bin/rakudobrew line 44.
    Use of uninitialized value $impl in string eq at /home/mvuets/.rakudobrew/bin/rakudobrew line 105.
    Use of uninitialized value $impl in exists at /home/mvuets/.rakudobrew/bin/rakudobrew line 109.
    Use of uninitialized value $impl in concatenation (.) or string at /home/mvuets/.rakudobrew/bin/rakudobrew line 110.
    Building  is NYI. Well volunteered!

Now it's more polite:

    $ rakudobrew build
    Build what?
    Available backends:
    parrot
    jvm
    moar
Add new sum-command 'available'

'available' lists options available to 'build'. For now it's only
backends. Coming soon: Rakudo versions.
Make 'available' list top-15 Rakudo versions

Naïve and dumb, but good enough for the first iteration.
BEWARE GitHub rate limits applied to unauthenticated API calls.
Tell users they can specify which Rakudo version to build

I peeked this undocumented feature from tadzik's Perl 6 Advent Calendar
2014 post. (Which is not published yet, hence cannot provide a link to
the future.)
Expose 'triple' to users and set defaults

Huh, one more hidden gem (-:
Add 'self-upgrade' sub-command

Since the way to install rakudobrew is to clone its git repo,
self-upgrade is as simple as pulling that repo.
Add explicit exit()

So, you know, it's clear you ain't gonna do anything anymore.
Merge upstream into misc-ui-improvments

Conflicts:
	bin/rakudobrew
Rip out my 'available' implementation

JJ Merelo implemented it via `git ls-remote` which is more robust and
less dependable on dependencies (-:
Fix and enhance list-available

First off, the original implementation would not mark the current
version as it was suppose to do:

    $ ./bin/rakudobrew list
    * moar-2014.11
    $ ./bin/rakudobrew list-available | grep '2014\.11'
      2014.11

Second, it does not make much sense, because Rakudo version is a
secondary aspect of a Perl 6 version. Primary one is backend.

So I fixed both, now it lists backends and Rakudo versions, and marks
the current combination with "*"'s like so:

    $ ./bin/rakudobrew current
    Currently running moar-2014.11
    $ ./bin/rakudobrew list-available | grep '\*'
    * moar
    * 2014.11