Remove support for deprecated v1 API

[sprin]

Fixes #7.

Removed from client:

• start_authenticate_v1
• finish_authenticate_v1
• CredentialsV1

Removed lxml as dep, v1 tests, and references in readme.

Ran tests (thank you for the awesome test suite!):

Name Stmts Miss Cover Missing

box 3 0 100%

box.client 216 0 100%

TOTAL 219 0 100%

Ran 51 tests in 0.039s

[tals]

Hey there, thank you for the diff. However:

• There are merge conflicts (you forked from an older version)
• v1 auth still works :)

[sprin]

Closing since v1 auth is still around, if unofficially: http://developers.box.com/v1-authentication/

Thanks for the great library!

[tals]

:+1: