user can use api_token to authenticate himself

User can choose his favorite authentication methods:
- username and password
- api_token provided by toggl

by default it uses username and password

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
default authentication with api_token

for security reasons prefer api_token authentication

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
Merge pull request #20 from FedericoVaga/master

[2] authentication with api_token