1.7.8 Faster UniFrac, focus dependencies, rm picante dependency

CHANGES IN VERSION 1.7.8
-------------------------

USER-VISIBLE CHANGES

	- Package dependencies reduced/clarified:
		https://github.com/joey711/phyloseq/issues/259
		Should reduce chances for collisions with other packages, and related issues.
		Removed any dependencies on the picante package.

	- Replaced picante::node.age() with a faster implementation, node_ages()
		Appears to be 3 times faster.
		Speeds up UniFrac() and tip_glom() calculations.