Add ability to login to internal Salesforce development instances

[elilevine]

First of all, this is a kick-ass tool! Saw it used during the Platform keynote at Dreamforce this year and was really impressed.

This issue is primarily for Salesforce devs who want to use force with non-public instances of Salesforce. Requirements:

• configurable server name
• configurable port
• configurable client id
• ability to authenticate against a server presenting a self-signed cert

[elilevine]

This pull request adds a new "dev" endpoint: force login dev

When using this endpoint, user must have the following environment variables set:

• FORCE_DEV_SERVER
• FORCE_DEV_HTTPS_PORT
• FORCE_DEV_CLIENT_ID

Dev server is allowed to present a self-signed certificate. Other endpoints still require a signed cert from the Salesforce server.

[ddollar]

Awesome. Going to close this issue in favor of keeping the conversation on #40