Add a reply-to field to emails

Emails can now specify a parent email ID which the new email is a reply
to.
Issue #789: Rename average price -> center price in order book printer
Merge branch 'develop' of github.com:BitShares/bitshares_toolkit into develop
Test margin calls
Update BUILD_UBUNTU.md

Add qttools5-dev to Ubuntu build instructions
Fix crash on start when node_config.json exists without private key
Merge pull request #802 from bitcoiners/patch-2

Update BUILD_UBUNTU.md
updating account meta info
Merge branch 'develop' of github.com:BitShares/bitshares_toolkit into develop
paying cancel fee with funds from canceled order
reduce bit asset share supply on cover
Update submodules
Merge branch 'develop' into btsx

Conflicts:
	programs/qt_wallet
	programs/web_wallet
Add fork for fixing market-issued asset supply
Remove btsx_bots submodule
Merge branch 'develop' into btsx
Revert "Remove btsx_bots submodule"

This reverts commit b5245e5fb559fee61c512db0316ccd260ec72afc.
increase the number of scanning threads from 1 to 4 in the event a wallet has more than one account
Merge branch 'develop' of github.com:BitShares/bitshares_toolkit into develop
Update readability of code, removed magic number