Support building a shared library
Merge pull request #13 from jbergstroem/shared

cmake: Support building a shared library
Add `UCL_PARSER_NO_TIME` flag to skip time parsing

Issue: #12
Reported by: @mitchellh
Update API documentation.
No more `static inline` public API functions.
Add `ucl_parser_add_string` function.

Issue:	#11
Reported by:	@mitchellh
Fix tests, as empty string is *NOT* a valid UCL.
Add `ucl_object_pop_key` functions.
Update minor version to 0.2.9.
Interface for objects validation.
Start ucl schema implementation.
Start schema validation implementation.
Add validator for UCL_OBJECT type.
Add validators for primitive types.
Add binary tree algorithms to test array schemas.
Implement arrays validation.
Add a simple test for schema.

Tests are based on:
https://github.com/json-schema/JSON-Schema-Test-Suite/
Fix test suite for schema.
Add schema tests to `run-test` target.
Add tests suite for schema.