Improve doc comment for Client.GetPage method.
Remove return from Client.Logout method

The return value should never be used, and it is impossible for the
method to return anything but true anyway.
Get rid of the Page type.

The Page type was useless, so GetPage now returns two strings (one for
the page body, one for the timestamp) instead of a *Page (and of course
an error).
Rename maxlag related variable in Client.call

The "num" variable has been renamed "retryAfter" to improve readability.
Overhaul API error handling

Add ExtractAPIErrors function which extracts API errors and warnings, if
any, from a *simplejson.Json object and returns them stored in a
multierror.Errors object from the github.com/joeshaw/multierror package.

ExtractAPIErrors will probably complain if the API response contains a
malformed warning. Although this is unlikely to happen, I should
probably fix it.

ErrorCheck is now deprecated and will be removed in a later commit.
GetCheck and PostCheck are removed entirely in this commit.

TODO: Testing... this code really needs to be tested more thoroughly.
Update doc comment for Logout.

Logout doesn't return anything anymore, and this commit updates the doc
comment to reflect that.
Remove ErrorCheck and related code from GetToken

ErrorCheck has been deprecated.
Remove deprecated func ErrorCheck
Add doc comment for ExtractAPIErrors