add placeholder text to search field

so people know that they have to add it in such a format
linking find out "more" link to docs
tightening up visual spacing and colors
adding primary buttons to multi-currency

using bootstrap classes to make our buttons look like buttons
tightening up the send form
Merge branch 'multi-currency' of https://github.com/stellar/stellar-client into multi-currency-design

Conflicts:
	app/styles/multi-currency.scss
	app/templates/gateways.html
	app/templates/manage-currencies.html
Merge pull request #728 from stellar/multi-currency-design

Multi-currency design pass