Refactor the Manifests class to an immutable

[westonal]

In line with objects should be immutable, refactor the Manifests class to be an immutable.

Acceptance criteria:

• Manifests is immutable, that is no exposed methods will change its internal state
• Manifests no longer implements Map<String, String>, though it should keep get method
• Manifests provides a mutable map copy from a method: Map<string, string> asMap() providing an upgrade route for anyone relying on its previous implementation of Map
• Remove @SuppressWarnings("PMD.TooManyMethods") from the class, as most of the old Map methods will be gone

[dmarkov]

@yegor256 please pay attention to this issue (par.21)

[dmarkov]

@westonized thanks for this report, I added 15 mins to your account, in transaction AP-92K25680T0309344B

[proshin-roman]

@westonal @dmarkov the issue has been fixed in a pull request #43. Please check if the ticket can be closed now.