Clean up compiler warnings

[kohlschuetter]

My Eclipse IDE is full of liqp warnings, making it hard to see the actual problems. Let's clean these up a bit.

Clean up code in src/main/java

• Infer generic parameters for Map, List, etc.
• Cease use of deprecated Java API.
• Suppress "unchecked", "hiding" warnings where appropriate.
• Add missing serialVersionUID where necessary.
• Use getRenderSettings()/getParseSettings() instead of deprecated access.
• Remove unused class members.
• Remove unused imports.

Clean up code in src/main/test

• Replace deprecated org.junit.Assert.assertThat with org.hamcrest.MatcherAssert.assertThat.
• Organize imports.
• Replace HashMap subclass with Collections.singletonMap where possible.
• Add SupressWarnings "unused", "deprecation", "serial" where appropriate.
• DateTest: Add additional checks for vararg method.

This change is rebased upon https://github.com/bkiers/Liqp/pull/274 and https://github.com/bkiers/Liqp/pull/269/commits

[msangel]

Merged there: https://github.com/bkiers/Liqp/pull/280

[msangel]

My opinions:

• Infer generic parameters for Map, List, etc. ✔️
• Cease use of deprecated Java API. ✔️
• Suppress "unchecked", "hiding" warnings where appropriate. ✔️
• Add missing serialVersionUID where necessary. ✔️ for adding, ❌ for generated (should be 1L )
• Use getRenderSettings()/getParseSettings() instead of deprecated access. ✔️
• Remove unused class members. ✔️
• Remove unused imports. ✔️
• Replace wildcard imports with explicit ❌
• Static imports for variables ❌
• Static imports for methods ✔️

Clean up code in src/main/test

• Replace deprecated org.junit.Assert.assertThat with org.hamcrest.MatcherAssert.assertThat. ✔️
• Organize imports. ✔️
• Replace HashMap subclass with Collections.singletonMap where possible. ✔️
• Add SupressWarnings "unused", "deprecation", "serial" where appropriate. ✔️
• DateTest: Add additional checks for vararg method. ✔️