Prepare v0.4.0 release

[falko17]

• README and crate-level docs have been modified to refer to the new 0.4.0 version.
• derive_builder dependency was updated, and base64 dependency was removed (the place where a base64-encoded string has been used was replaced with a hexadecimal-encoded string).
• One new clippy warning has been addressed (comparison of references replaced with direct comparison).

After this PR is merged, version 0.4.0 will be released.

[coveralls]

Pull Request Test Coverage Report for Build 4532382635

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 90.226%

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Files with Coverage Reduction	New Missed Lines	%
src/common/scope/mod.rs	3	92.13%
<!--	Total:	3		-->

Totals
Change from base Build 3991589354:	-0.2%
Covered Lines:	1597
Relevant Lines:	1770

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💛 - Coveralls