Add contests, register and unregister commands

[gnojus]

This PR adds ability to list upcoming contests, and register/unregister from them.

[xalanq]

Wow, awesome feature!

[xalanq]

Emm.. I found you have imported a new library goquery. Is it necessary ? Can regexp do the same works? I just don't want to import too many things..

[gnojus]

goquery is also imported here:

cf-tool/client/watch.go

Line 16 in 3c4e2e6

"github.com/PuerkitoBio/goquery"

.
Everything can be done with standard libraries, the question is the how much code readability are we willing to sacrifice?
Though if you really want I could probably strip out goquery from this PR.

[xalanq]

goquery is also imported here:

cf-tool/client/watch.go

Line 16 in 3c4e2e6

"github.com/PuerkitoBio/goquery"

.
Everything can be done with standard libraries, the question is the how much code readability are we willing to sacrifice?
Though if you really want I could probably strip out goquery from this PR.

Hahahah. I forgot I used that lib too. OK, just use it.