Add remove_assets_from_site method

[gschneider-r7]

Description

Adds a new method to the Connection class to remove site association from a collection of assets. Note that this can result in assets being deleted if they either only belong to 1 site or if asset linking is disabled.

Usage:

nsc = Nexpose::Connection.new('localhost', 'myuser', 'mypassword')
# login, list sites and assets, etc.

assets = [25, 72] # array of asset ID(s) is required
site_id = 3
nsc.remove_assets_from_site(assets, site_id)

Motivation and Context

Resolves #228

How Has This Been Tested?

Tested on a console with asset linking enabled where a collection of assets are associated to 2 sites.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• I have updated the documentation accordingly (if changes are required).
• I have added tests to cover my changes.
• All new and existing tests passed.

[gschneider-r7]

I think the Travis-CI failures are due to this ku1ik/rainbow#44

Will look at it some more later.

[gschneider-r7]

Still something up with the travis builds on 2.1.5, so I'll have to look at fixing up the dependencies. The other builds are currently passing so it's not necessarily a blocker to merging this PR.