Add network proportion to homepage and /vspinfo (and revoked proportion)

[jholdstock]

Proportion is calculated using the number of tickets currently registered with the VSP, divided by the total size of the network ticket pool as reported by getblockheader.

The value will only ever be an estimate because:

• it's possible for a single ticket to be added to multiple VSPs.
• vspd does not distinguish between immature and live tickets, whereas getblockheader only reports live tickets.
• getblockheader is reporting the size of the ticket pool as of the previous block, not the current block.

Closes #206

[xaur]

If it will only ever be an estimate, how about reflecting that in the field name? e.g. networkproportionestimate. That makes the name long though :S

"networkproportion":0.04847841472045294

I think 17 decimal places is an overkill. The lowest non-zero proportion is 1 / 50,000 = 0.00002, assuming the ticket pool ever reaches 50K. Let's take a less rounded number, 1 / 49,998 ~= 0.000020001 - still only 9 places.

[jholdstock]

I actually had all of those exact same thoughts myself. Eventually settled on using the shorter name rather than the more descriptive one, and used a float64 rather than a shorter float32 in the API simply to be more consistent with the other fields in the API.

I dont feel particularly strongly about either, and will happily change both to match your suggestions.

[jholdstock]

Because this is a rather small PR and it hasnt received a technical review yet, I've included another commit which also displays the number of revoked tickets as a percentage