Parses SessionIndex in the login response

[gtothesquare]

@tngan Not sure if this is the way to go, but It seems that samlify needs the SessionIndex to start a logout request. However, is not parse during the loginResponse. So, this PRs adds it.

[coveralls]

Coverage remained the same at 89.273% when pulling 0f67f25 on gtothesquare:session-index-parsing into 21b9035 on tngan:master.

[tngan]

Apparently, the purpose of session index is used to identify which session is being logged out, so that it's worth to include it during the parsing process, thanks @gtothesquare.