Add option for compressed brain wallet

[rapodaca]

From the "Brain Wallet" tab, entering a passphrase generates the address and WIF for an uncompressed public key only. Moreover, the compression status isn't noted on the screen.

This could be confusing to users who create key pairs using custom entropy sources. If they use another tool to double check, it may exclusively or by default show compressed WIF and address.

I'd like to see a checkbox labeled "Compressed" under the Brain Wallet tab. If checked, it produces a compressed address/WIF. It can be unchecked by default to maintain compatibility with current behavior.