Track volume gain command-line option, VoiceOver for Russian, Gentoo installation steps, tweaks

1. pico2wave doesn't support Russian language, so I've added handling of Russian VoiceOver via another text-to-speech engine: RHVoice (it seems to be one of the best tts programs for Russian available).
1. Added simple deletion of the whole VoiceOver directory before generation of new VoiceOver files.
1. Reasonable and comfortable track volume gain seems to differ from one person/headphones to another, so I've added simple command-line switch to easily tweak the gain value.
1. There are other minor fixes and README changes

Please, merge this pull request!
Thank you in advance!