Added support for HTTP methods

[mihirvj]

See #331

I think before we finalize the pythonic method for this, these changes can be utilized.

[coveralls]

Coverage decreased (-0.04%) to 92.12% when pulling 7a3a73c on mihirvj:master into 483546d on jakubroztocil:master.

[sigmavirus24]

Can you change the commit message (and title) to read:

"Added completion support for HTTP methods"

The message is very misleading otherwise.

[jkbrzt]

@mihirvj I'd also add PATCH, OPTIONS, and I believe there are other commonly used methods as well

[jkbrzt]

Now I realize this will only work without any --options (e.g. $ http -v P<tab> wont complete).

The more robust approaches mentioned in #331 are probably a better way to go.