Add possibility to specify a template URL

Required parameters should not contain square braces

Convert indention into spaces instead of tabs

Tiny style changes.

validator.rb: Print xcodebuild command on -v.

Proposed in https://github.com/CocoaPods/Specs/pull/9179.

Include declared_path in 'No podspec found..' error message for PathSource

Fixes #1778

Fixes #1821

Uniformize spec template quotes

Radically reduce first run pod setup bandwidth.

Given that Cocoapods doesn't need the history of the Specs repo we can radically improve the initial setup time by only checking out HEAD. This reduces the initial download from >200MB to roughly 3.5MB, a massive win for folks with slow interwebs.

git pull/git fetch+git rebase work as expected.