Add `get composertags` command

This adds a command to retrieve a list of tags for packages installed by composer and compare them against the installed version.

Current output:
```
monolog/monolog
1.7.0
1.6.0 <= Installed

twig/twig
v1.14.2 <= Installed

filp/whoops
1.0.9 <= Installed

mustache/mustache
v2.4.1
v2.4.0
v2.3.1
v2.3.0
v2.2.0
v2.1.0 <= Installed

symfony/http-foundation
v2.4.0-RC1
v2.4.0-BETA2
v2.4.0-BETA1
v2.3.7 <= Installed

psr/log
1.0.0
Installed: dev-master

elkuku/console-progressbar
1.0 <= Installed

elkuku/g11n
2.1 <= Installed

raveren/kint
v0.9 <= Installed

squizlabs/php_codesniffer
1.5.0RC4
1.5.0RC3
1.5.0RC2
1.5.0RC1
1.5.0 <= Installed

joomla/framework
1.0-beta3
1.0-beta2
1.0-beta
1.0-alpha
1.0
Installed: dev-master
```

Available options:

```
--all   Show all tags or only the most recent.
```

e.g.
```
$ ./cli/tracker.php get composertags --all
...
monolog/monolog
1.7.0
1.6.0 <= Installed
1.5.0
1.4.1
1.4.0
1.3.1
1.3.0
1.2.1
1.2.0
1.1.0
1.0.2
1.0.1
1.0.0-RC1
1.0.0
...
```
Note: This only works for projects hosted on GitHub (currently)