@LostTRoll
The reasons i level up Imbue first are mainly these:
1) 1st level of Shatter gives +10 armor aura, the following levels give only +5, while Imbue scales a bit better.
2) I am often playing in situations where people poke each other, where people get hurt on both sides. If i want to stay in lane, i have to heal myself and my carry - and to do that, i need enough mana to do it. Now if i have a weak Imbue, i need to use it more times to heal the same damage, meaning it will drain my mana more.
If someone has 20 armor + 10 from shatter aura, it gives 23% damage reduction.
Now if i put one more point into Shatter, it will give 20+15 ~ 26% damage reduction.
But if i put that point in Imbue, it would allow me to heal for 100hp instead of 60, for just a little more mana.

Unless you are faced against a heavy burst combo, i would prefer Imbue, since +5 armor for your carry per level of Shatter just feels too weak compared to better healing power.

@Yarda
The magic resistance marks are really nice, it's certainly a possibility and i will ad them to the build.

Playing a support means more than just sitting in the brush and popping heal every few seconds, and i think this guide shows it nicely.

About your runes though.. you get attack damage marks, that gives you arrond 0,65 mana per hit more than what you would get with different runes. I am sceptical about this, my choice would have been magic resistance runes, probably.

@LostTRoll
I think you should level Shatter and Imbue depending on the situation. If your carry is getting harrased to hell, get Imbue, else get Shatter. The armor aura is nice, but if you are at very little hp and can not afford to recall, some extra armor will not help too much...

i think you should prioritize shatter over heal xept at lvl 1-2 cuz then you add that extra armor to your lane partner and yourself so you'll live longer