Project structure update

For future updates is is required to update project structure.
Updated readme with verbose description of mutex modes and parameters.

In new projects consider using pkg/rwlock package instead of rwlock as it may be eventually removed.

Configurable reader lock mode

Added optional lock mode which changes the behavior of the reader lock.
In ModePreferWriter reader lock retreats if writer has placed an intention to acquire the lock.
In ModePreferReader reader lock ignores writer's intention and tries to acquire the lock.

Application can use both modes on single lock depending on the developer intention.
It is recommended to use ModePreferWriter in case when you have rare but very important writes. Otherwise prefer ModePreferReader.

Initial release with modules support

Fix #4

Remove v1 from module name.