Upgraded multipart-post 2.0 & databasedotcom to 2.0.0. Added changelog.

Upgrades multipart-post dependency to 2.0 for compatibility with other gems. Eg we couldn't use this gem with a recent version of zendesk_api because zendesk_api needs multipart-post v2.

Bumps version to 2.0.0 to because this is essentially an api change and semantic versioning makes users' lives easier.

Adds a changelog.