Echo Effect updates.

Add ramping to the beginning and end of delay buffer to prevent clicks.
Add Send parameter to control how much of dry is fed into the buffer.
Clamp echo buffer
Apply dry signal to wet portion to mimic send-effect behavior.
Change ordering of echo knobs.
Tweak dry/wet mixing to prevent overdriving of signal.
Merge pull request #269 from ywwg/echo-updates

Echo Effect updates.
Merge branch 'master' of https://github.com/mixxxdj/mixxx
remove left-alignment of library headings