[feature/tus-support] Support for Tus uploads [wip]

[felix-schwarz]

Description

This PR adds initial support for uploads via the tus.io protocol.

Related Issue

owncloud/ios-sdk#65

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

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[felix-schwarz]

With the merge in the SDK of feature/tus-support into develop, this PR is no longer needed. Further TUS protocol support development now takes places in the SDK develop branch and is tracked via owncloud/ios-sdk#65.